Until very recently [ when? ]  in  Taiwan , left-handed people were forced to switch to being right-handed, or at least switch to writing with the right hand. [ citation needed ]  Due to the importance of  stroke order , developed for the comfortable use of right-handed people, it is considered more difficult to write legible Chinese characters with the left hand than it is to write Latin letters, though difficulty is subjective and depends on the writer. [ 125 ] [ better source needed ]  Because writing when moving one's hand away from its side towards the other side of the body can cause smudging if the outward side of the hand is allowed to drag across the writing, writing in the  Latin alphabet  might possibly be less feasible with the left hand than the right under certain circumstances. Conversely, right-to-left alphabets, such as the Arabic and Hebrew, are generally considered easier to write with the left hand. [ citation needed ]  Depending on the position and inclination of the writing paper, and the writing method, the left-handed writer can write as neatly and efficiently or as messily and slowly as right-handed writers. Usually the left-handed child needs to be taught how to write correctly with the left hand, since discovering a comfortable left-handed writing method on one's own may not be straightforward. [ 126 ] [ 127 ]


In the  Soviet school system , all left-handed children were forced to write with their right hand. [ 128 ] [ 129 ]


International Left-Handers Day is held annually every August 13. [ 130 ]  It was founded by the Left-Handers Club in 1992, with the club itself having been founded in 1990. [ 130 ]  International Left-Handers Day is, according to the club, "an annual event when left-handers everywhere can celebrate their sinistrality (left-handedness) and increase public awareness of the advantages and disadvantages of being left-handed." [ 130 ]  It celebrates their uniqueness and differences, who are from seven to ten percent of the world's population. Thousands of left-handed people in today's society have to adapt to use right-handed tools and objects. Again according to the club, "in the U.K. alone there were over 20 regional events to mark the day in 2001 – including left-v-right sports matches, a left-handed tea party, pubs using left-handed corkscrews where patrons drank and played pub games with the left hand only, and nationwide 'Lefty Zones' where left-handers' creativity, adaptability and sporting prowess were celebrated, whilst right-handers were encouraged to try out everyday left-handed objects to see just how awkward it can feel using the wrong equipment." [ 130 ]


Kangaroos  and other  macropod   marsupials  show a left-hand preference for everyday tasks in the wild. 'True' handedness is unexpected in marsupials however, because unlike  placental mammals , they lack a  corpus callosum . Left-handedness was particularly apparent in the red kangaroo ( Macropus rufus ) and the eastern gray kangaroo ( Macropus giganteus ). Red-necked (Bennett's) wallabies ( Macropus rufogriseus ) preferentially use their left hand for behaviours that involve fine manipulation, but the right for behaviours that require more physical strength. There was less evidence for handedness in  arboreal  species. [ 131 ]  Studies of dogs, horses, and domestic cats have shown that females of those species tend to be right-handed, while males tend to be left-handed. [ 132 ]


Hyper–Rayleigh scattering optical activity  ( / ˈ r eɪ l i /   RAY -lee ), a form of  chiroptical harmonic scattering , is a  nonlinear optical  physical effect whereby chiral scatterers (such as nanoparticles or molecules) convert  light  (or other  electromagnetic radiation ) to higher frequencies via  harmonic generation  processes, in a way that the intensity of generated light depends on the chirality of the scatterers. "Hyper–Rayleigh scattering" is a nonlinear optical counterpart to  Rayleigh scattering . " Optical activity " refers to any changes in light properties (such as intensity or polarization) that are due to  chirality .


The effect was theoretically predicted in 1979, [ 1 ]  in a mathematical description of hyper Raman scattering optical activity. Within this theoretical model, upon setting the initial and final frequencies of light to the same value, the mathematics describe the hyper Rayleigh scattering optical activity. The theory was well in advance of its time, and the effect remained elusive for 40 years. Its author  David L. Andrews  referred to it as the "impossible theory". However, in January 2019, an experimental demonstration was reported by Ventsislav K. Valev and his team. [ 2 ] [ 3 ]  The team investigated the hyper Rayleigh scattering (at the  second harmonic generation  frequency) from chiral nanohelices made of silver. Valev and his team observed that the intensity of the hyper Rayleigh scattering light depended on the direction of circularly polarized light and that this dependence reversed with the chirality of the nanohelices. Valev's work unambiguously established that the effect is physically possible, opening the way for nonlinear chiroptical investigations of a variety of chiral light-scattering materials; including molecules, [ 4 ]  plasmonic metal nanoparticles [ 5 ]  and semiconductor nanoparticles. [ 6 ]


Hyper Rayleigh scattering optical activity (HRS OA) is arguably the most fundamental nonlinear chiral optical (chiroptical) effect; since other nonlinear chiroptical effects have additional requirements, which make them conceptually more involved, i.e. less fundamental. HRS OA is a scattering effect and therefore it does not require the frequency conversion process to be coherent, contrary to other nonlinear chiroptical effects, such as second harmonic generation circular dichroism [ 7 ]  or second harmonic generation optical rotation. [ 8 ]  Moreover, HRS OA is a  parametric process : the initial and final quantum mechanical states of the excited electron are the same. Because the excitation proceeds via  virtual states , there is no restriction on the frequency of incident light. By contrast, other nonlinear scattering effects, such as  two-photon circular dichroism  and hyper-Raman are non-parametric: they require real energy states that restrict the frequencies at which these effects can be observed.


Soon after the first demonstration of hyper Rayleigh scattering optical activity in metal nanoparticles, [ 3 ]  the effect was replicated in organic molecules, specifically aromatic oligoamide foldamers. [ 4 ]


Whereas the initial experimental demonstration of hyper-Rayleigh scattering optical activity was observed at the second harmonic of the illumination frequency of light, the effect is general and can be observed at higher harmonics. The first demonstration of hyper-Rayleigh scattering optical activity at the third harmonic was reported by Valev's team in 2021, from silver nanohelices. [ 9 ] [ 10 ]


In  chemistry ,  inherent chirality  is a property of asymmetry in molecules arising, not from a stereogenic or chiral center, but from a twisting of the molecule in 3-D space.   The term was first coined by Volker Boehmer in a 1994 review, to describe the chirality of  calixarenes  arising from their non-planar structure in 3-D space.


This phenomenon was described as resulting from "the absence of a place of symmetry or an inversion center in the molecule as a whole". [ 1 ]  Boehmer further explains this phenomenon by suggesting that if an inherently chiral calixarene macrocycle were opened up it would produce an "achiral linear molecule". [ 1 ]  There are two commonly used notations to describe a molecules inherent chirality: cR/cS (arising from the notation used for classically chiral compounds, with  c  denoting curvature) and P/M. [ 2 ] [ irrelevant citation ]  Inherently chiral molecules, like their classically chiral counterparts, can be used in chiral host–guest chemistry, enantioselective synthesis, and other applications. [ 3 ]   There are naturally occurring inherently chiral molecules as well.  Retinal, a chromophore in  rhodopsin . exists in solution as a racemic pair of enantiomers due to the curvature of an achiral polyene chain. [ 4 ]


After creating a series of traditionally chiral  calixarenes  (through the addition of a chiral substituent group on the top or bottom rim of the macrocycle,) the first inherently chiral calixarenes were synthesized in 1982, though the molecules were not yet described as such.  The inherently chiral calixarenes featured an XXYZ or WXYZ substitution pattern, such that the planar representation of the molecule does not show any chirality, and if the macrocycle were to be broken open, this would produce an achiral linear molecule. [ 5 ]   The chirality in these calixarenes is instead derived from the curvature of the molecule in space. [ 6 ]


Due to the initial lack of a formal definition after the initial conception, the term inherent chirality was utilized to describe a variety of chiral molecules that don't fall into other defined chirality types.  The first fully formulated definition of inherent chirality was published in 2004 by Mandolini and Schiaffino, (and later modified by Szumna): [ 4 ]


inherent chirality arises from the introduction of a curvature in an ideal planar structure that is devoid of perpendicular symmetry planes in its bidimensional representation.


Inherent chirality has been known by a variety of names in the literature including bowl chirality (in fullerene fragments), intrinsic chirality, helicity (see section 3a) residual enantiomers (as applied to sterically hindered molecular propellers,) and cyclochirality (though this is often considered to be a more specific example and cannot be applied to all inherently chiral molecules). [ 4 ]


A simple example of inherent chirality is that of  corannulene  commonly referred to as "bowl chirality" in the literature.  The chirality of an unsubstituted corranulene (containing no classic stereogenic centers) cannot be seen in a 2D representation, but becomes clear when a 3D representation is evoked, as the C 5  symmetry of corranulenes provides the molecules with a source of chirality (figure 2.)  Racemization of these molecules is possible through an inversion of curvature, though some inherently chiral molecules have inversion barriers comparable to a classic chiral center. [ 4 ]


Some inherently chiral molecules contain  chirality planes , or planes within a given molecules across which the molecule is dissymmetric.  Paracyclophanes often contain chiral planes if the bridge across the phenylene unit is short enough, or if the phenylene contains another substituent, not in the bridge, that hinders rotation of the phenylene unit.


Similar to chirality planes,  chirality axes  arise from an axis about which the spatial arrangement of substituents creates chirality.  This can be seen in helical molecules (see section 3a) as well as some alkenes.


Spiro compounds  (compounds with a twisted structure of two or more rings) can have inherent chirality at the spiroatom, due to the twisting of the achiral ring system.


Inherently chiral alkenes have been synthesized through the use of a "buckle" where in an achiral, linear alkene is forced into a chiral conformation.  Alkenes have no classical chirality, so generally, an external stereogenic center must be introduced.  However, by locking the alkene into a conformation through the use of an achiral buckle allows for the creation of an inherently chiral alkene.  Inherently chiral alkenes have been synthesized through the use of dialkoxysilanes, with a large enough racemization barrier that enantiomers have been isolated. [ 7 ]


The term  laterality  refers to the preference most  humans  show for one side of their  body  over the other. Examples include  left-handedness/right-handedness  and left/right- footedness ; it may also refer to the primary use of the left or right hemisphere in the brain. It may also apply to animals or plants. The majority of tests have been conducted on humans, specifically to determine the effects on  language .


Most humans are  right-handed . Many are also right-sided in general (that is, they prefer to use their right  eye , right  foot  and right  ear  if forced to make a choice between the two). The reasons for this are not fully understood, but it is thought that because the left  cerebral hemisphere  of the  brain  controls the right side of the body, the right side is generally stronger; it is suggested that the  left cerebral hemisphere  is dominant over the right in most humans because in 90–92% of all humans, the left hemisphere is the  language  hemisphere.


Human  cultures  are predominantly right-handed, and so the right-sided trend may be socially as well as biologically enforced. This is quite apparent from a quick survey of  languages . The  English  word "left" comes from the  Anglo-Saxon  word  lyft  which means "weak" or "useless". Similarly, the  French  word for left,  gauche , is also used to mean "awkward" or "tactless", and  sinistra , the  Latin  word from which the English word "sinister" was derived, means "left". Similarly, in many cultures the word for "right" also means "correct". The English word "right" comes from the Anglo-Saxon word  riht  which also means "straight" or "correct."


This linguistic and social bias is not restricted to  European cultures : for example,  Chinese characters  are designed for right-handers to write, and no significant left-handed culture has ever been found in the world.


When a person is forced to use the hand opposite of the hand that they would naturally use, this is known as  forced laterality , or more specifically  forced dextrality . A study done by the Department of Neurology at  Keele University ,  North Staffordshire Royal Infirmary  suggests that forced dextrality may be part of the reason that the percentage of left-handed people decreases with the higher age groups, both because the effects of pressures toward right-handedness are cumulative over time (hence increasing with age for any given person subjected to them) and because the prevalence of such pressure is decreasing, such that fewer members of younger generations face any such pressure to begin with. [ 1 ]


Ambidexterity  is when a person has approximately equal skill with both  hands  and/or both sides of the body. True ambidexterity is very rare. Although a small number of people can write competently with both hands and use both sides of their body well, even these people usually show preference for one side of their body over the other. However, this preference is not necessarily consistent for all activities. Some people may, for instance, use their right hand for  writing , and their  left hand  for playing  racket sports  and  eating [ 2 ]  ( see also:   cross-dominance ).


Also, it is not uncommon that people  preferring to use the right hand prefer to use the left leg , e.g. when using a shovel, kicking a ball, or operating control pedals. In many cases, this may be because they are disposed for left-handedness but have been trained for right-handedness, which is usually attached to learning and behavioural disorders (term usually so called as " cross dominance "). [ 3 ]  In the sport of  cricket , some players may find that they are more comfortable  bowling  with their left or right hand, but  batting  with the other hand.


Approximate statistics, complied in 1981, are given below: [ 4 ]


Laterality of  motor  and sensory control has been the subject of a recent intense study and review. [ 5 ]  It turns out that the hemisphere of speech is the hemisphere of action in general and that the command hemisphere is located either in the right or the left hemisphere (never in both). Around 80% of people are left hemispheric for speech and the remainder are right hemispheric: ninety percent of right-handers are  left hemispheric for speech , but only 50% of left-handers are right hemispheric for speech (the remainder are left hemispheric). The  reaction time  of the neurally dominant side of the body (the side opposite to the major hemisphere or the command center, as just defined) is shorter than that of the opposite side by an interval equal to the interhemispheric transfer time. Thus, one in five persons has a handedness that is the opposite for which they are wired (per laterality of command center or brainedness, as determined by reaction time study mentioned above).


Cerebral dominance  or  specialization  has been studied in relation to a variety of human functions. With speech in particular, many studies have been used as evidence that it is generally localized in the  left hemisphere . Research comparing the effects of  lesions  in the two hemispheres,  split-brain  patients, and  perceptual asymmetries  have aided in the knowledge of speech lateralization. In one particular study, the left hemisphere's sensitivity to differences in rapidly changing  sound cues  was noted (Annett, 1991). This has real world implication, since very fine  acoustic discriminations  are needed to comprehend and produce  speech signals . In an  electrical stimulation  demonstration performed by Ojemann and Mateer (1979), the exposed  cortex  was  mapped  revealing the same cortical sites were activated in  phoneme discrimination  and mouth movement sequences (Annett, 1991).


As suggested by Kimura (1975, 1982), left hemisphere speech lateralization might be based upon a preference for movement sequences as demonstrated by  American Sign Language  (ASL) studies. Since ASL requires intricate  hand movements for language communication , it was proposed that skilled hand motions and speech require sequences of action over time. In  deaf  patients with a left hemispheric  stroke  and damage, noticeable losses in their abilities to sign were noted. These cases were compared to studies of normal speakers with  dysphasias  located at lesioned areas similar to the deaf patients. In the same study, deaf patients with right hemispheric lesions did not display any significant loss of signing nor any decreased capacity for motor sequencing (Annett, 1991).


One theory, known as the acoustic laterality theory, the physical properties of certain speech sounds are what determine laterality to the left hemisphere.  Stop consonants , for example t, p, or k, leave a defined  silent period  at the end of words that can easily be distinguished. This theory postulates that changing sounds such as these are preferentially processed by the left hemisphere. As a result of the right ear being responsible for transmission to sounds to the left hemisphere, it is capable of perceiving these sounds with rapid changes. This right ear advantage in hearing and speech laterality was evidenced in  dichotic listening studies .  Magnetic imaging  results from this study showed greater left hemisphere activation when actual words were presented as opposed to pseudowords. [ 6 ]  Two important aspects of  speech recognition  are  phonetic cues , such as format patterning, and  prosody  cues, such as  intonation ,  accent , and emotional state of the speaker (Imaizumi, Koichi, Kiritani, Hosoi & Tonoike, 1998).


In a study done with both  monolinguals  and  bilinguals , which took into account language experience,  second language proficiency , and onset of bilingualism among other variables, researchers were able to demonstrate left hemispheric dominance. In addition, bilinguals that began speaking a second language early in life demonstrated bilateral hemispheric involvement. The findings of this study were able to predict differing patterns of cerebral language lateralization in adulthood (Hull & Vaid, 2006).


It has been shown that cerebral lateralization is a widespread phenomenon in the  animal kingdom . [ 7 ]  Functional and structural differences between left and right brain hemispheres can be found in many other vertebrates and also in invertebrates. [ 8 ]


It has been proposed that negative, withdrawal-associated emotions are processed predominantly by the right hemisphere, whereas the left hemisphere is largely responsible for processing positive, approach-related emotions. This has been called the "laterality- valence  hypothesis". [ 9 ]


One sub-set of laterality in animals is limb dominance. Preferential limb use for specific tasks has been shown in species including chimpanzees, mice, bats, wallabies, parrots, chickens and toads. [ 8 ]


Another form of laterality is hemispheric dominance for processing conspecific vocalizations, reported for chimpanzees, sea lions, dogs, zebra finches and Bengalese finches. [ 8 ]


In mice ( Mus musculus ), laterality in paw usage has been shown to be a learned behavior (rather than inherited), [ 10 ]  due to which, in any population, half of the mice become left-handed while the other half becomes right-handed. The learning occurs by a gradual reinforcement of randomly occurring weak asymmetries in paw choice early in training, even when training in an unbiased world. [ 11 ] [ 12 ]  Meanwhile, reinforcement relies on short-term and long-term memory skills that are strain-dependent, [ 11 ] [ 12 ]  causing strains to differ in the degree of laterality of its individuals. Long-term memory of previously gained laterality in handedness due to training is heavily diminished in mice with absent corpus callosum and reduced hippocampal commissure. [ 13 ]  Regardless of the amount of past training and consequent biasing of paw choice, there is a degree of randomness in paw choice that is not removed by training, [ 14 ]  which may provide adaptability to changing environments.


Domestic horses  ( Equus caballus ) exhibit laterality in at least two areas of neural organization, i.e. sensory and motor. In  thoroughbreds , the strength of motor laterality increases with age. Horses under 4 years old have a preference to initially use the right nostril during olfaction. [ 15 ]  Along with olfaction, French horses have an eye laterality when looking at novel objects. There is a correlation between their score on an emotional index and eye preference; horses with higher emotionality are more likely to look with their left eye. The less emotive French saddlebreds glance at novel objects using the right eye, however, this tendency is absent in the  trotters , although the emotive index is the same for both breeds. [ 16 ]   Racehorses  exhibit laterality in stride patterns as well. They use their preferred stride pattern at all times whether racing or not, unless they are forced to change it while turning, injured, or fatigued. [ 17 ]


In  domestic dogs  ( Canis familiaris ), there is a correlation between motor laterality and noise sensitivity - a lack of paw preference is associated with noise-related fearfulness. (Branson and Rogers, 2006) [ citation needed ]  Fearfulness is an undesirable trait in guide dogs, therefore, testing for laterality can be a useful predictor of a successful guide dog. Knowing a guide dog's laterality can also be useful for training because the dog may be better at walking to the left or the right of their blind owner. [ 18 ]


Domestic cats  ( Felis catus ) show an individual handedness when reaching for static food. In one study, 46% preferred to use the right paw, 44% the left, and 10% were ambi-lateral; 60% used one paw 100% of the time. There was no difference between male and female cats in the proportions of left and right paw preferences. In moving-target reaching tests, cats have a left-sided behavioural asymmetry. [ 19 ]  One study indicates that laterality in this species is strongly related to temperament. Furthermore, individuals with stronger paw preferences are rated as more confident, affectionate, active, and friendly. [ 20 ]


Chimpanzees  show right-handedness in certain conditions. This is expressed at the population level for females, but not males. The complexity of the task has a dominant effect on handedness in chimps. [ 21 ]


Cattle  use  visual/brain lateralisation  in their visual scanning of novel and familiar stimuli. [ 22 ]  Domestic cattle prefer to view novel stimuli with the left eye, (similar to horses, Australian magpies, chicks, toads and fish) but use the right eye for viewing familiar stimuli. [ 23 ]


Schreibers' long-fingered bat  is lateralized at the population level and shows a left-hand bias for climbing or grasping. [ 24 ]


Some types of  mastodon  indicate laterality through the fossil remains having differing tusk lengths. [ citation needed ]


Marsupials  are fundamentally different from other mammals in that they lack a  corpus callosum . [ 25 ]  However, wild  kangaroos  and other  macropod   marsupials  have a left-hand preference for everyday tasks. Left-handedness is particularly apparent in the  red kangaroo  ( Macropus rufus ) and the  eastern gray kangaroo  ( Macropus giganteus ). The  red-necked wallaby  ( Macropus rufogriseus ) preferentially uses the left hand for behaviours that involve fine manipulation, but the right for behaviours that require more physical strength. There is less evidence for handedness in  arboreal  species. [ 26 ]


Parrots  tend to favor one foot when grasping objects (for example fruit when feeding). Some studies indicate that most parrots are left footed. [ 27 ]


The  Australian magpie  ( Gymnorhina tibicen ) uses both left-eye and right-eye laterality when performing anti-predator responses, which include  mobbing . Prior to withdrawing from a potential predator, Australian magpies view the animal with the left eye (85%), but prior to approaching, the right eye is used (72%). The left eye is used prior to jumping (73%) and prior to circling (65%) the predator, as well as during circling (58%) and for high alert inspection of the predator (72%). The researchers commented that "mobbing and perhaps circling are agonistic responses controlled by the LE[left eye]/right hemisphere, as also seen in other species. Alert inspection involves detailed examination of the predator and likely high levels of fear, known to be right hemisphere function." [ 28 ]


Yellow-legged gull  ( Larus michahellis ) chicks show laterality when reverting from a supine to prone posture, and also in pecking at a dummy parental bill to beg for food. Lateralization occurs at both the population and individual level in the reverting response and at the individual level in begging. Females have a leftward preference in the righting response, indicating this is sex dependent. Laterality in the begging response in chicks varies according to laying order and matches variation in egg  androgens  concentration.
 [ 29 ]


Laterality determines the organisation of rainbowfish ( Melanotaenia  spp.) schools. These fish demonstrate an individual eye preference when examining their reflection in a mirror. Fish which show a right-eye preference in the mirror test prefer to be on the left side of the school. Conversely, fish that show a left-eye preference in the mirror test or were non-lateralised, prefer to be slightly to the right side of the school. The behaviour depends on the species and sex of the school. [ 30 ]


Three species of toads, the  common toad  ( Bufo bufo ),  green toad  ( Bufo viridis ) and the  cane toad  ( Bufo marinus ) show stronger escape and defensive responses when a model predator was placed on the toad's left side compared to their right side. [ 31 ]   Emei music frogs  ( Babina daunchina ) have a right-ear preference for positive or neutral signals such as a conspecific's advertisement call and white noise, but a left-ear preference for negative signals such as predatory attack. [ 32 ]


The  Mediterranean fruit fly  ( Ceratitis capitata ) exhibits left-biased population-level lateralisation of aggressive displays (boxing with forelegs and wing strikes) with no sex-differences. [ 33 ]  In ants,  Temnothorax albipennis  (rock ant) scouts show behavioural lateralization when exploring unknown nest sites, showing a population-level bias to prefer left turns. One possible reason for this is that its environment is partly maze-like and consistently turning in one direction is a good way to search and exit mazes without getting lost. [ 34 ]  This turning bias is correlated with slight asymmetries in the ants' compound eyes (differential ommatidia count). [ 35 ]


Left-hand traffic  ( LHT ) and  right-hand traffic  ( RHT ) are the practices, in  bidirectional traffic , of keeping to the left side and to the right side of the road, respectively. They are fundamental to  traffic flow , and are sometimes called the  rule of the road . [ 1 ]  The terms right- and left-hand  drive  refer to the position of the driver and the  steering wheel  in the vehicle and are, in automobiles, the reverse of the terms right- and left-hand  traffic . The rule also includes where on the road a vehicle is to be driven, if there is room for more than one vehicle in one direction, and the side on which the vehicle in the rear overtakes the one in the front. For example, a driver in an LHT country would typically overtake on the right of the vehicle being overtaken.


RHT is used in 165 countries and territories, mainly in the  Americas ,  Continental Europe , most of  Africa  and mainland  Asia  (except  South Asia ), while 75 countries use LHT, [ 2 ]  which account for about a sixth of the world's land area, a quarter of its roads, and about a third of its population. [ 3 ]  In 1919, 104 of the world's territories were LHT and an equal number were RHT. Between 1919 and 1986, 34 of the LHT territories switched to RHT. [ 4 ]


While many of the countries using LHT were part of the  British Empire , others such as  Indonesia ,  Japan ,  Nepal ,  Bhutan ,  Macao ,  Thailand ,  Mozambique  and  Suriname  were not, as well as formerly,  Sweden  and  Iceland , which have used RHT since  September 1967  and  late May 1968  respectively. Most of the countries that were part of the  French colonial empire  adopted RHT.


Historical switches of traffic handedness have often been motivated by factors such as changes in political administration, a desire for uniformity within a country or with neighboring states, or availability and affordability of vehicles.


In LHT, traffic keeps left and cars usually have the steering wheel on the right (RHD: right-hand drive) and  roundabouts  circulate clockwise. RHT is the opposite: traffic keeps right, the driver usually sits on the left side of the car (LHD: left-hand drive), and roundabouts circulate anticlockwise.


In most countries, rail traffic follows the handedness of the roads; but many of the countries that switched road traffic from LHT to RHT did not switch their trains. Boat traffic on  bodies of water  is RHT, regardless of location. Boats are traditionally piloted from the  starboard  side (and not the port side like RHT road traffic vehicles) to facilitate  priority to the right .


Historically, many places kept left, while many others kept right, often within the same country. There are many myths that attempt to explain why one or the other is preferred. [ 5 ]  About 90 percent of people are  right-handed , [ 6 ]  and many explanations reference this. Horses are traditionally mounted from the left, and led from the left, with the reins in the right hand. So people walking horses might use RHT, to keep the animals separated. Also referenced is the need for pedestrians to keep their swords in the right hand and pass on the left as in LHT, for self-defence. It has been suggested that wagon-drivers whipped their horses with their right hand, and thus sat on the left-hand side of the wagon, as in RHT. Academic Chris McManus notes that writers have stated that in 1300,  Pope Boniface VIII  directed pilgrims to keep left; others suggest that he directed them to keep to the right, and there is no documented evidence to back either claim. [ 5 ]


The UK introduced LHT in the  East Africa Protectorate  (present-day  Kenya ), the  Protectorate of Uganda ,  Tanganyika  (formerly part of  German East Africa ; present-day  Tanzania ),  Rhodesia  (present-day  Zambia / Zimbabwe ),  Eswatini  and the  Cape Colony  (present-day  South Africa  and  Lesotho ), as well as in  British West Africa  (present-day Ghana, Gambia, Sierra Leone and Nigeria); [ 7 ]  former British West Africa, however, has now switched to RHT, as all its neighbours, which are former French colonies, use RHT. South Africa, formerly the Cape Colony, introduced LHT in former  German South West Africa , present-day  Namibia , after the end of  World War I .


Sudan, formerly part of  Anglo-Egyptian Sudan , switched to RHT in 1973.  Most of its neighbours were RHT countries, with the exception of Uganda and Kenya, but since the independence of  South Sudan  in 2011, all of its neighbours drive on the right (including South Sudan, despite its land borders with two LHT countries). [ 8 ]


Although  Portugal  switched to RHT in 1928, its  colony of Mozambique  remained LHT because it has land borders with former British colonies (with LHT).


France introduced RHT in  French West Africa  and the  Maghreb , [ citation needed ]  where it is still used. Countries in these areas include  Mali ,  Mauritania ,  Ivory Coast ,  Burkina Faso ,  Benin ,  Niger ,  Morocco ,  Algeria , and  Tunisia . Other French former colonies that are RHT include  Cameroon ,  Central African Republic ,  Chad ,  Djibouti ,  Gabon , and the  Republic of the Congo .


Rwanda  and  Burundi  are RHT but are considering switching to LHT (see "Potential future shifts" section below).


In the late 18th century, right-hand traffic started to be introduced in the United States based on  teamsters ' use of large freight wagons pulled by several pairs of horses and without a driver's seat; the (typically right-handed)  postilion  held his whip in his right hand and thus sat on the left rear horse, and therefore preferred other wagons passing on the left so that he would have a clear view of other vehicles. [ 9 ] [ better source needed ]  The first keep-right law for  driving in the United States  was passed in 1792 and applied to the  Philadelphia and Lancaster Turnpike . [ 10 ]   Massachusetts  formalized RHT in 1821. [ 11 ]  However, the  National Road  was LHT until 1850, "long after the rest of the country had settled on the keep-right convention". [ 12 ]  Today the United States is RHT except the  United States Virgin Islands , [ 13 ]  which is LHT like many neighbouring islands.


Some special-purpose vehicles in the United States, like certain postal service trucks, garbage trucks, and parking-enforcement vehicles, are built with the driver's seat on the right for safer and easier access to the curb. A common example is the  Grumman LLV , which is used nationwide by the  US Postal Service  and by  Canada Post .


In Canada, the provinces of  Quebec  and  Ontario  were always RHT because they were created out of the former French colony of  New France . [ 14 ]  The province of  British Columbia  changed to RHT in stages from 1920 to 1923, [ 15 ] [ 16 ]   New Brunswick ,  Nova Scotia , and  Prince Edward Island  in 1922, 1923, and 1924 respectively, [ 17 ]  and the  Dominion of Newfoundland  (part of Canada since 1949) [ 18 ]  in 1947, in order to allow traffic (without side switch) to or from the United States. [ 19 ]


In the  West Indies , colonies and territories drive on the same side as their parent countries, except for the  United States Virgin Islands . Many of the island nations are former British colonies and drive on the left, including  Jamaica ,  Antigua and Barbuda ,  Barbados ,  Dominica ,  Grenada ,  Saint Kitts and Nevis ,  Saint Lucia ,  Saint Vincent and the Grenadines ,  Trinidad and Tobago , and  The Bahamas . However, most vehicles in The Bahamas, [ 20 ]   Cayman Islands , [ 21 ]   Turks and Caicos Islands [ 22 ]  and both the  British Virgin Islands , [ 23 ]  and the  United States Virgin Islands  are LHD due to their being imported from the United States. [ 23 ]


Brazil , a Portuguese colony until the early 19th century, had in the 19th and the early 20th century mixed rules, with some regions still on LHT, switching these remaining regions to RHT in 1928, the same year Portugal switched sides. [ 24 ]  Other Central and South American countries that later switched from LHT to RHT include Argentina, Chile, Panama, [ 25 ]  Paraguay, [ 26 ]  and Uruguay.


Suriname , along with neighbouring  Guyana , are the only two remaining LHT countries in  South America . [ 27 ]


LHT was introduced by the UK in  British India  (now  India ,  Pakistan ,  Myanmar , and  Bangladesh ),  British Malaya  and  British Borneo  (now  Malaysia , Brunei and Singapore), as well as  British Hong Kong . These countries, except Myanmar, are still LHT, as well as neighbouring countries  Bhutan  and  Nepal . Myanmar switched to RHT in 1970, [ 28 ]  although much of its infrastructure is still geared to LHT as its neighbours India, Bangladesh and  Thailand  use LHT. Most cars are used RHD vehicles imported from Japan. [ 29 ]   Afghanistan  was LHT until the 1950s, in line with Pakistan (former part of British India). [ 30 ]


Although  Portuguese Timor  (present-day  East Timor ), which shares the island of  Timor  with  Indonesia , who is LHT, switched to RHT with  Portugal  in 1928, [ 1 ]  it switched back to LHT in 1976 during the  Indonesian occupation of East Timor .


In the 1930s, parts of  China  such as the  Shanghai International Settlement ,  Canton  and Japanese-occupied  northeast China  used LHT. However, in 1946 the  Republic of China  made RHT mandatory in  China  (including  Taiwan ). Taiwan was LHT under  Japanese colonization  from 1895–1945.  Portuguese Macau  (present-day  Macau ) remained LHT, along with  British Hong Kong , despite being transferred to China in 1999 and 1997 respectively.


Both  North Korea  and  South Korea  use RHT since 1946, after liberation from  Japanese colonialization . [ 31 ]


The  Philippines  was mostly LHT during its  Spanish [ 32 ]  and  American  colonial periods, [ 33 ] [ 34 ]  as well as during the  Commonwealth era . [ 35 ]  During the  Japanese occupation , the Philippines remained LHT, [ 36 ]  as was required by the Japanese; [ 37 ]  but during the  Battle of Manila , the liberating American forces drove their tanks to the right for easier facilitation of movement. RHT was formalized in 1945 through a decree by president  Sergio Osmeña . [ 38 ]  Even though RHT was formalized, RHD vehicles such as public buses were still imported into the Philippines until a law passed banning the importation of RHD vehicles except in special cases. These RHD vehicles are required to be converted to LHD. [ 39 ]


Japan was never part of the British Empire, but its traffic also drives on the left. Although this practice goes back to the Edo period (1603–1868), it was not until 1872 – the year Japan's first railway was introduced, built with technical aid from the British – that this unwritten rule received official acknowledgment. Gradually, a massive network of railways and tram tracks was built, with all railway vehicles driven on the left-hand side. However, it took another half-century, until 1924, until left-hand traffic was legally mandated. Post- World War II   Okinawa  was ruled by the  United States Civil Administration of the Ryukyu Islands  until 1972, and was RHT until  6 a.m. the morning of 30 July 1978, when it switched back to LHT . [ 40 ]  The conversion operation was known as 730 ( Nana-San-Maru , which refers to the date of the changeover). Okinawa is one of only a few places to have changed from RHT to LHT in the late 20th century. While Japan drives on the left and most Japanese vehicles are RHD, imported vehicles (e.g. BMW, Mercedes-Benz, Porsche) are generally bought as LHD since LHD cars are considered to be status symbols. [ 41 ]


Vietnam  became RHT as part of  French Indochina , as did  Laos  and  Cambodia . In Cambodia, RHD cars, many of which were smuggled from Thailand, were banned in 2001, even though they accounted for 80% of vehicles in the country. [ 42 ]


In a study of the ancient traffic system of  Pompeii , Eric Poehler was able to show that drivers of carts drove in the middle of the road whenever possible. This was the case even on roads wide enough for two lanes. [ 43 ] : 136   The wear marks on the kerbstones, however, prove that when there were two lanes of traffic, and the volume of traffic made it necessary to divide the lanes, the drivers always drove on the right-hand side. [ 43 ] : 150–155   These considerations can also be demonstrated in the archaeological findings of other cities in the  Roman Empire . [ 43 ] : 218–219


One of the first references in England to requiring traffic direction was an order by the London  Court of Aldermen  in 1669, requiring a man to be posted on  London Bridge  to ensure that "all cartes going to keep on the one side and all cartes coming to keep on the other side". [ 44 ]  It was later legislated as the  London Bridge Act 1756  ( 29 Geo. 2  c. 40), which required that "all carriages passing over the said bridge from London shall go on the east side thereof" – those going south to remain on the east, i.e. the left-hand side by direction of travel. [ 45 ]  This may represent the first statutory requirement for LHT. [ 46 ]


In the  Kingdom of Ireland , a law of 1793 (33 Geo. 3. c. 56 (I)) provided a ten- shilling  fine to anyone not driving or riding on the left side of the road within the  county of the city  of  Dublin , and required the local road overseers to erect written or printed notices informing road users of the law. [ 47 ]  The  Road in Down and Antrim Act 1798  (38 Geo. 3. c. 28 (I)) required drivers on the road from Dublin to  Donadea  to keep to the left. This time, the punishment was ten shillings if the offender was not the owner of the vehicle, or one  Irish pound  (twenty shillings) if he/she was. [ 48 ]  The  Grand Juries (Ireland) Act 1836  ( 6 & 7 Will. 4  c. 116) mandated LHT for the whole country, violators to be fined up to five shillings and imprisoned in default for up to one month. [ 49 ]


An oft-repeated story is that  Napoleon  changed the custom from LHT to RHT in France and the countries he conquered after the  French Revolution . Scholars who have looked for documentary evidence of this story have found none, and contemporary sources have not surfaced, as of 1999. [update] [ 4 ]  In 1827, long after Napoleon's reign, Edward Planta wrote that, in  Paris , "The coachmen have no established rule by which they drive on the right or left of the road, but they cross and jostle one another without ceremony." [ 50 ]


Rotterdam had no fixed rules until 1917, [ 51 ]  although the rest of the Netherlands was RHT. In May 1917 the police in Rotterdam ended traffic chaos by enforcing right hand traffic.


In Russia, in 1709, the Danish envoy under Tsar  Peter the Great  noted the widespread custom for traffic in Russia to pass on the right, but it was only in 1752 that  Empress Elizabeth  officially issued an edict for traffic to keep to the right. [ 52 ]


After the  Austro-Hungarian Empire  broke up, the resulting countries gradually changed to RHT. In  Austria ,  Vorarlberg  switched in 1921, [ 53 ]   North Tyrol  in 1930,  Carinthia  and  East Tyrol  in 1935, and the rest of the country in 1938. [ 54 ]  In  Romania ,  Transylvania , the  Banat  and  Bukovina  were LHT until 1919, while  Wallachia  and  Moldavia  were already RHT.  Partitions of Poland  belonging to the  German Empire  and the  Russian Empire  were RHT, while the former  Austrian Partition  changed in the 1920s. [ 55 ]  Croatia-Slavonia switched on joining the  Kingdom of Yugoslavia  in 1918, although  Istria  and  Dalmatia  were already RHT. [ 56 ]  The  switch in Czechoslovakia  from LHT to RHT had been planned for 1939, but was accelerated by the start of the  German occupation of Czechoslovakia  that year. [ 57 ]  Similarly, Hungary switched in 1941. West Ukraine was LHT, but the rest of Ukraine, having been part of the Russian Empire, was RHT. [ citation needed ]


In Italy, it had been decreed in 1901 that each  province  define its own  traffic code , including the handedness of traffic, [ 58 ]  and the 1903  Baedeker  guide reported that the rule of the road varied by region. [ 5 ]  For example, in  Northern Italy , the provinces of  Brescia ,  Como ,  Vicenza , and  Ravenna  were RHT while nearby provinces of  Lecco ,  Verona , and  Varese  were LHT, [ 58 ]  as were the cities  Milan ,  Turin , and  Florence . [ 5 ]  In 1915,  allied  forces of  World War I  imposed LHT in areas of military operation, but this was revoked in 1918.  Rome  was reported by  Goethe  as LHT in the 1780s.  Naples  was also LHT although surrounding areas were often RHT. In cities, LHT was considered safer since pedestrians, accustomed to keeping right, could better see oncoming vehicular traffic. [ 58 ]  Finally, in 1923  Benito Mussolini  decreed that all LHT areas would gradually transition to RHT. [ 58 ]  In spite of this, some Italian heavy commercial vehicles were right-hand drive until the traffic code was changed in 1959. [ citation needed ]


Portugal switched to RHT in 1928. [ 1 ]


Finland , formerly part of LHT Sweden, switched to RHT in 1858 as the  Grand Duchy of Finland  by Russian decree. [ 59 ]


Spain  switched to RHT in 1918, but not in the entire country. In  Madrid  people continued to drive on the left until 1924 when a national law forced drivers in Madrid switch to RHT. [ 60 ]   Madrid Metro  still uses LHT.


Sweden  switched to RHT in 1967, having been LHT from about 1734 [ 61 ]  despite having land borders with RHT countries  Norway  and Finland, and approximately 90% of cars being left-hand drive (LHD). [ 62 ]   A referendum in 1955  overwhelmingly rejected a change to RHT, but, a few years later, the government ordered it and it occurred on Sunday, 3 September 1967 [ 63 ]  at 5 am. The accident rate then dropped sharply, [ 64 ]  but soon rose to near its original level. [ 65 ]  The day was known as Högertrafikomläggningen, or  Dagen H  for short.


When Iceland switched to RHT the following year, it was known as  Hægri dagurinn  or  H-dagurinn  ("The H-Day"). [ 66 ]  Most passenger cars in Iceland were already LHD.


The United Kingdom is LHT, but two of its  overseas territories ,  Gibraltar  and the  British Indian Ocean Territory , are RHT. In the late 1960s, the British  Department for Transport  considered switching to RHT, but declared it unsafe and too costly for such a built-up nation. [ 67 ]  Road building standards, for motorways in particular, allow asymmetrically designed road junctions, where merge and diverge lanes differ in length. [ 68 ]


Today, four countries in Europe continue to use LHT, all island nations: the United Kingdom,  Ireland  (formerly part of the UK),  Cyprus  and  Malta  (both former British colonies).


Many former British colonies in the region have always been LHT, including Australia, New Zealand,  Fiji ,  Kiribati ,  Solomon Islands ,  Tonga , and  Tuvalu ; and nations that were previously administered by Australia:  Nauru  and  Papua New Guinea .


Initially traffic was slow and very sparse, but, as early as 1856, a newspaper said, "The cart was near to the right hand kerb. According to the rules of the road, it should have been on the left side. In turning sharp round a right-hand corner, a driver should keep away to the opposite side." That rule was codified when the first  Highway Code  was written in 1936. [ 69 ]


Samoa , a former German colony, had been RHT for more than a century, but switched to LHT in 2009, [ 70 ]  making it the first territory in almost 30 years to change sides. [ 71 ]  The move was legislated in 2008 to allow Samoans to use cheaper vehicles imported from Australia, New Zealand, or Japan, and to harmonise with other South Pacific nations. A political party,  The People's Party , was formed by the group People Against Switching Sides (PASS) to protest against the change, with PASS launching a legal challenge; [ 72 ]  in April 2008 an estimated 18,000 people attended demonstrations against switching. [ 73 ]  The motor industry was also opposed, as 14,000 of Samoa's 18,000 vehicles were designed for RHT and the government refused to meet the cost of conversion. [ 71 ]  After months of preparation, the switch from right to left happened in an atmosphere of national celebration. There were no reported incidents. [ 3 ]  At 05:50 local time, Monday 7 September, a radio announcement halted traffic, and an announcement at 6:00 ordered traffic to switch to LHT. [ 70 ]  The change coincided with more restrictive enforcement of speeding and seat-belt laws. [ 74 ]  That day and the following were declared public holidays, to reduce traffic. [ 75 ]  The change included a three-day ban on alcohol sales, while police mounted dozens of checkpoints, warning drivers to drive slowly. [ 3 ]


Rwanda  and  Burundi , former Belgian colonies in  Central Africa , are RHT but are considering switching to LHT [ 76 ] [ 77 ]  like neighbouring members of the  East African Community  (EAC). [ 78 ]  A survey in 2009 found that 54% of Rwandans favoured the switch. Reasons cited were the perceived lower costs of RHD vehicles, easier maintenance and the political benefit of harmonising traffic regulations with other EAC countries. The survey indicated that RHD cars were 16% to 49% cheaper than their LHD counterparts. [ 79 ]  In 2014, an internal report by consultants to the Ministry of Infrastructure recommended a switch to LHT. [ 80 ]  In 2015, the ban on RHD vehicles was lifted; RHD trucks from neighbouring countries cost $1,000 less than LHD models imported from Europe. [ 81 ] [ 82 ]


Although many LHT jurisdictions are on islands, there are cases where vehicles may be driven from LHT across a border into a RHT area. Such borders are mostly located in Africa and southern Asia. The  Vienna Convention on Road Traffic  regulates the use of foreign registered vehicles in the 78 countries that have ratified it.


LHT Thailand has three RHT neighbours: Cambodia, Laos, and Myanmar. Most of its borders use a simple traffic light to do the switch, but there are also interchanges that enable the switch while keeping up a continuous flow of traffic. [ 83 ]


There are six road border crossing points between Hong Kong and mainland China. In 2006, the daily average number of vehicle trips recorded at  Lok Ma Chau  was 31,100. [ 84 ]  The next largest is  Man Kam To , where there is no changeover system and the border roads on the mainland side  Wenjindu  intersect as one-way streets with a main road.


The  Takutu River Bridge  (which links LHT Guyana and RHT Brazil [ 85 ] ) is the only border in the Americas where traffic changes sides.


In RHT jurisdictions, vehicles are typically configured as  left hand drive  (LHD), with the  steering wheel  on the left side of the passenger compartment. In LHT jurisdictions, the reverse is true as the  right hand drive  (RHD) configuration. In most jurisdictions, the position of the steering wheel is not regulated, or explicitly permitted to be anywhere. [ 86 ]  The driver's side, the side closer to the centre of the road, is sometimes called the  offside , while the passenger side, the side closer to the side of the road, is sometimes called the  nearside . [ 87 ]


Most  windscreen wipers  are preferentially designed to better clean the driver's side of the  windscreen  and thus have a longer wiper blade on the driver's side and wipe up from the passenger side to the driver's side. Thus on LHD configurations, they wipe up from right to left, viewed from inside the vehicle, and do the opposite on RHD vehicles. [ citation needed ]


In both LHD and RHD vehicles, gear shifters are in the same position, and the shift patterns are not reversed.


Historically there was less consistency in the relationship of the position of the driver to the handedness of traffic. Most American cars produced before 1910 were RHD. [ 10 ]  In 1908  Henry Ford  standardised the  Model T  as LHD in RHT America, [ 10 ]  arguing that with RHD and RHT, the passenger was obliged to "get out on the street side and walk around the car" and that with steering from the left, the driver "is able to see even the wheels of the other car and easily avoids danger." [ 88 ]  By 1915 other manufacturers followed Ford's lead, due to the popularity of the Model T. [ 10 ]


In specialised cases, the driver will sit on the nearside, or curbside. Examples include:


Generally, the convention is to mount a motorcycle on the left, [ 90 ]  and  kickstands  are usually on the left [ 91 ]  which makes it more convenient to mount on the safer kerbside [ 91 ]  as is the case in LHT. Some jurisdictions prohibit fitting a  sidecar  to a motorcycle's offside. [ 92 ] [ 93 ]


In 2020, there were 160 LHD heavy goods vehicles in the UK involved in accidents (5%) for a total of 3,175 accidents, killing 215 people (5%) for a total of 4271. [ 94 ]


It has been suggested that right-hand drive vehicles, and hence the left-hand traffic direction, are associated with greater safety. As most drivers are right-handed, the dominant right hand remains controlled on the steering wheel while the non-dominant left hand can manipulate gears. [ 95 ]  The right field of vision may also be more dominant, thereby permitting a superior view of oncoming traffic.


Some manufacturers primarily produce left-hand drive vehicles, due to the larger or nearer market for such vehicles. For such models supplied to left-hand traffic markets, in the right-hand drive configuration, the manufacturer may reuse the same  dashboard  configuration as is used in the left-hand drive models, with the steering column and pedals moved to the right-hand side. Oft-used controls (such as audio volume and fan controls) that were placed near the left-hand driver for ease of access, are now situated on the far side of the  center console  for the right-hand driver. This may make them more difficult to reach quickly or without looking away from the road ahead.


In some cases, the manufacturer's dashboard design incorporates blanks and modular components, which permits the controls and underlying electronics to be rearranged to suit the right-hand drive model. This may be done in the factory, after import, or as an after-market modification.


Most low-beam headlamps produce an asymmetrical light suitable for use on only one side of the road. Low beam headlamps in LHT jurisdictions throw most of their light forward-leftward; those for RHT throw most of their light forward-rightward, thus illuminating obstacles and road signs while minimising glare for oncoming traffic.


In Europe, headlamps approved for use on one side of the road must be adaptable to produce adequate illumination with controlled glare for temporarily driving on the other side of the road, [ 96 ] : p.13 ¶5.8  . This may be achieved by affixing masking strips or prismatic lenses to a part of the lens or by moving all or part of the headlamp optic so all or part of the beam is shifted or the asymmetrical portion is occluded. [ 96 ] : p.13 ¶5.8.1   Some varieties of the  projector-type headlamp  can be fully adjusted to produce a proper LHT  or  RHT beam by shifting a lever or other movable element in or on the lamp assembly. [ 96 ] : p.12 ¶5.4   Some vehicles adjust the headlamps automatically when the car's  GPS  detects that the vehicle has moved from LHT to RHT and vice versa. [ citation needed ]


In Europe since early 1980s, [ 97 ]  cars must be equipped with one or two red  rear fog lamps . A single rear fog lamp must be located between the vehicle's longitudinal centreline and the outer extent of the driver's side of the vehicle. [ 98 ]


ANCAP  reports that some RHD cars imported to Australia did not perform as well on crash tests as the LHD versions, although the cause is unknown, and may be due to differences in testing methodology. [ 99 ]


In most countries rail traffic travels on the same side as road traffic. However, there are many instances of railways built using LHT British technology which remained LHT despite their nations' road traffic becoming RHT. Examples include:  Argentina ,  Belgium ,  Bolivia ,  Cambodia ,  China ,  Egypt ,  France ,  Iraq ,  Israel ,  Italy ,  Laos ,  Monaco ,  Morocco ,  Myanmar ,  Nigeria ,  Peru ,  Portugal ,  Senegal ,  Slovenia ,  Sweden ,  Switzerland ,  Taiwan ,  Tunisia ,  Uruguay  and  Venezuela . France is mainly LHT for trains except for the classic lines in  Alsace–Lorraine , [ 100 ]  which were converted from LHT to RHT under German administration from 1870 to 1918. In North America, multi-track rail lines with  centralized traffic control  are typically signaled to allow operation on any track in both directions, and the side of operation will vary based on the railroad's specific operational requirements. [ 101 ]  In practice however, rail traffic is more often RHT.  Indonesia  is the only country in the world which has RHT for rails (even for newer rail systems such as the LRT and the MRT systems) and LHT for roads.


Metro and light rail sides of operation vary and might not match railways or roads in their country. Some systems where the metro matches the side of the national rail network but not the roads include those in  Bilbao ,  Buenos Aires ,  Cairo ,  Catania ,  Jakarta ,  Lisbon ,  Lyon ,  Naples , and  Rome . A small number of cities, including  Madrid  and  Stockholm , originally ran on the same side as road traffic when the systems opened in 1919 and 1950 respectively, but had road traffic change in 1924 and 1967 respectively. Conversely, metros in France (except for the aforementioned Lyon) and mainland China run on the right just like roads, while mainline trains run on the left.


A small number of systems have situational reasons to differ from the norm. On the  MTR  in Hong Kong, the section originally known as the  Ma On Shan line  (now part of the Tuen Ma line) runs on the right to make interchanging with the East Rail line easier, while the rest of the system runs on the left. On the  Seoul Metropolitan Subway , lines that integrate with Korail (except Line 3, which is disconnected from the rest of the network) run on the left, while the lines that are not run on the right. In  Nizhny Novgorod , Line 2 runs on the left due to the track layout when it first opened as a branch of Line 1. In  Lima , Line 1 runs entirely on the left, while Line 2 runs entirely on the right.


Metro Line M1 in Budapest  is the only metro line to have switched sides. It originally ran on the left but switched to right hand-running during the line's reconstruction around 1973.


Because trams frequently operate on roads, they generally operate on the same side as other road traffic.


Boats are traditionally piloted from  starboard  (the right-hand side) to facilitate  priority to the right .
According to the  International Regulations for Preventing Collisions at Sea , water traffic is effectively RHT: a vessel proceeding along a narrow channel must keep to starboard, and when two power-driven vessels are meeting head-on both must alter course to starboard also.


Typically, especially for larger vessels, a radio call will be made between two vessels, or with a  Vessel Traffic Service (VTS)  to co-ordinate if the vessels will pass "green-to-green" or "red-to-red". Marine traffic uses a system of green lighting for the starboard (right-hand) side and red for port (left-hand) side: to pass "green-to-green" the green (starboard, right-hand) side of the vessels will pass each other, essentially being left-hand traffic. Similarly, passing "red-to-red" means the red (port, left-hand) side of the vessels will pass each other, forming right-hand traffic.


In busy waterways, directional shipping lanes may be set up to facilitate handedness of traffic. For example, the  Strait of Dover  (Pas-de-Calais) on the  English Channel  uses RHT with North Sea-bound vessels following the French coast and Atlantic-bound vessels following the English coast.


For aircraft the US  Federal Aviation Regulations  suggest RHT principles, both in the air and on water, and in aircraft with side-by-side cockpit seating, the pilot-in-command (or more senior flight officer) traditionally occupies the left seat. [ 102 ]  However, helicopter practice tends to favour the right hand seat for the pilot-in-command, particularly when flying solo. [ 103 ]


Of the 195 countries currently recognised by the  United Nations , 141 use RHT and 54 use LHT on roads in general.


A country and its territories and dependencies are counted as one. Whichever directionality is listed first is the type that is used in general in the traffic category.


According to the  Vienna Convention on Road Traffic , which mostly covers Europe, if having a vehicle registered and legal to drive in one of the Convention countries, it is legal to drive it in any other of the countries, for visits and first year of residence after moving. This is regardless of whether it fulfils all the rules of the visitor countries. This convention does not affect rules on usage or registration of local vehicles.


Mefloquine , sold under the brand name  Lariam  among others, is a  medication  used to prevent or treat  malaria . [ 4 ]  When used for prevention it is typically started before potential exposure and continued for several weeks after potential exposure. [ 4 ]  It can be used to treat mild or moderate malaria but is not recommended for severe malaria. [ 4 ]  It is taken  by mouth . [ 4 ]


Common side effects include vomiting, diarrhea, headaches, sleep disorders, and a rash. [ 4 ]  Serious side effects include potentially long-term mental health problems such as  depression ,  hallucinations , and  anxiety  and neurological side effects such as  poor balance ,  seizures , and  ringing in the ears . [ 4 ]  It is therefore not recommended in people with a history of mental health problems or  epilepsy . [ 4 ]  It appears to be safe during  pregnancy  and  breastfeeding . [ 1 ]


Mefloquine was developed by the United States Army in the 1970s and came into use in the mid-1980s. [ 5 ] [ 6 ] [ 7 ]  It is on the  World Health Organization's List of Essential Medicines . [ 8 ] [ 9 ]  It is available as a  generic medication . [ 4 ]


Mefloquine is used to both prevent and treat certain forms of malaria. [ 10 ]


Mefloquine is useful for the prevention of malaria in all areas except for those where parasites may have  resistance  to multiple medications, [ 11 ]  and is one of several anti-malarial medications recommended by the United States  Centers for Disease Control and Prevention  for this purpose. It is also recommended by the  Infectious Disease Society of America  for malaria prophylaxis as a first or second-line agent, depending on resistance patterns in the malaria found in the geographic region visited. [ 12 ] [ 13 ]  It is typically taken for one to two weeks before entering an area with malaria. [ 10 ]   Doxycycline  and  atovaquone/proguanil  provide protection within one to two days and may be better tolerated. [ 14 ] [ 15 ]  If a person becomes ill with malaria despite prophylaxis with mefloquine, the use of  halofantrine  and  quinine  for treatment may be ineffective. [ 16 ] : 4


Mefloquine is used as a treatment for  chloroquine -sensitive or resistant  Plasmodium falciparum  malaria, and is deemed a reasonable alternative for uncomplicated chloroquine-resistant  Plasmodium vivax  malaria. [ 10 ] [ 16 ]  It is one of several drugs recommended by the United States' Centers for Disease Control and Prevention. [ 17 ]
It is not recommended for severe malaria infections, particularly infections from  P. falciparum , which should be treated with intravenous  antimalarials . [ 10 ] [ 16 ]  Mefloquine does not eliminate parasites in the liver phase of the disease, and people with  P. vivax  malaria should be treated with a second drug that is effective for the liver phase, such as  primaquine . [ 16 ] : 4


Resistance  to mefloquine is common around the west border in  Cambodia  and other parts of Southeast Asia. [ 18 ]  The mechanism of resistance is by increase in  Pfmdr1  copy number. [ 19 ]


Common side effects include vomiting, diarrhea, headaches, and a rash. [ 4 ]  Severe side effects requiring hospitalization are rare, [ 11 ]  but include mental health problems such as  depression ,  hallucinations ,  anxiety  and neurological side effects such as  poor balance ,  seizures , and  ringing in the ears . [ 4 ]  Mefloquine is therefore not recommended in people with a history of  psychiatric disorders  or  epilepsy . [ 4 ]


In 2013, the U.S.  Food and Drug Administration  (FDA) added a  boxed warning  to the  prescription label  of mefloquine regarding the potential for neuropsychiatric side effects that may persist even after discontinuing administration of the medication. [ 20 ] [ 21 ]  In 2013 the FDA stated "Neurologic side effects can occur at any time during drug use, and can last for months to years after the drug is stopped or can be permanent." [ 22 ]  Neurologic effects include  dizziness , loss of balance,  seizures , and  tinnitus . Psychiatric effects include  nightmares ,  visual hallucinations ,  auditory hallucinations ,  anxiety ,  depression , unusual behavior, and  suicidal ideations .


Central nervous system events requiring hospitalization occur in about one in 10,000 people taking mefloquine for malaria prevention, with milder events (e.g., dizziness, headache,  insomnia , and vivid dreams) in up to 25%. [ 23 ]  When some measure of subjective severity is applied to the rating of adverse events, about 11–17% of travelers are incapacitated to some degree. [ 14 ]


Mefloquine may cause abnormalities with heart rhythms that are visible on  electrocardiograms . Combining mefloquine with other drugs that cause similar effects, such as quinine or  quinidine , can increase these effects. Combining mefloquine with halofantrine can cause significant increases in  QTc intervals . [ 16 ] : 10


Mefloquine is contraindicated in those with a previous history of seizures or a recent history of psychiatric disorders. [ 10 ]


Available data suggests that mefloquine is safe and effective for use by pregnant women during all trimesters of pregnancy, [ 24 ]  and it is widely used for this indication. [ 25 ]  In pregnant women, mefloquine appears to pose minimal risk to the fetus, [ 25 ] [ 26 ]  and is not associated with increased risk of birth defects or miscarriages. [ 27 ]  Compared to other malaria chemoprophylaxis regimens, however, mefloqinone may produce more side effects in non-pregnant travelers.


Mefloquine is also safe and effective for use during breastfeeding, [ 24 ]  though it appears in breast milk in low concentrations. [ 11 ] [ 16 ] : 9   The  World Health Organization  (WHO) gives approval for the use of mefloquine in the second and third trimesters of pregnancy and use in the first trimester does not mandate termination of pregnancy. [ 11 ]


Mefloquine is metabolized primarily through the liver. Its elimination in persons with impaired liver function may be prolonged, resulting in higher plasma levels and an increased risk of adverse reactions. The mean elimination plasma half-life of mefloquine is between two and four weeks. Total clearance is through the liver, and the primary means of excretion is through the bile and feces, as opposed to only 4% to 9% excreted through the urine. During long-term use, the plasma half-life remains unchanged. [ 28 ] [ 29 ]


Liver function tests should be performed during long-term administration of mefloquine. [ 30 ]  Alcohol use should be avoided during treatment with mefloquine. [ 31 ]


Specifically it is used as mefloquine hydrochloride.


Mefloquine is a  chiral  molecule with two  asymmetric carbon  centres, which means it has four different  stereoisomers . The drug is currently manufactured and sold as a  racemate  of the ( R , S )- and ( S , R )-enantiomers by  Hoffmann-La Roche , a Swiss  pharmaceutical  company. Essentially, it is two drugs in one. Plasma concentrations of the (–)-enantiomer are significantly higher than those for the (+)-enantiomer, and the pharmacokinetics between the two enantiomers are significantly different. The (+)-enantiomer has a shorter half-life than the (–)-enantiomer. [ 14 ]


Mefloquine was formulated at  Walter Reed Army Institute of Research  (WRAIR) in the 1970s shortly after the end of the Vietnam war. Mefloquine was number 142,490 of a total of 250,000 antimalarial compounds screened during the study. [ 5 ]


Mefloquine was the first Public-Private Venture (PPV) between the US Department of Defense and a pharmaceutical company. WRAIR transferred all its phase I and phase II clinical trial data to Hoffman-LaRoche and  Smith Kline . FDA approval as a treatment for malaria was swift. Most notably, phase III safety and tolerability trials were skipped. [ 5 ]


The drug was first approved in Switzerland in 1984 by Hoffmann-LaRoche, [ 32 ]  who brought it to market with the name  Lariam . [ 33 ]


However, mefloquine was not approved by the FDA for prophylactic use until 1989. This approval was based primarily on compliance, while safety and tolerability were overlooked. [ 5 ]  Because of the drug's very long half-life, the  Centers for Disease Control  originally recommended a mefloquine dosage of 250 mg every two weeks; however, this caused an unacceptably high malaria rate in the  Peace Corps  volunteers who participated in the approval study, so the drug regimen was switched to once a week. [ 14 ]


By 1991, Hoffman was marketing the drug on a worldwide basis. [ 33 ]


By the 1992  UNITAF , Canadian soldiers were being prescribed the drug  en masse . [ 34 ]


By 1994, medical professionals were noting "severe psychiatric side effects observed during prophylaxis and treatment with mefloquine", and recommending that "the absence of  contraindications  and minor side effects during an initial course of mefloquine should be confirmed before another course is prescribed." [ 35 ]  Other doctors at the  University Hospital of Zurich  noted in a case of "a 47-year-old, previously healthy Japanese tourist" who had severe neuropsychiatric side-effects from the drug that [ 36 ]


The neuropsychiatric side effects of the antimalarial drug mefloquine are well documented. They include anxiety, depression, hallucinations, acute psychosis, and seizures. The incidence of these side effects is 1 in 13,000 with prophylactic use and 1 in 250 with therapeutic use.


The first randomized, controlled trial on a mixed population was performed in 2001. Prophylaxis with mefloquine was compared to prophylaxis with  atovaquone-proguanil . Roughly 67% of participants in the mefloquine arm reported greater than or equal to one adverse event, versus 71% in the atovaquone-proguanil arm. In the mefloquine arm, 5% of the users reported severe events requiring medical attention, versus 1.2% in the atovaquone-proguanil arm. [ 5 ] [ 37 ]


In August 2009, Roche stopped marketing Lariam in the United States. [ 38 ]


Retired soldier  Johnny Mercer , who was later appointed Minister for Veterans Affairs by  Boris Johnson , told in 2015 that he had received "a letter about once or twice a week" about ill-effects from the drug. [ 39 ]  In July 2016, Roche took this brand off the market in Ireland. [ 38 ]


In 2006, the Australian military deemed mefloquine "a third-line drug" alternative, and over the five years from 2011 only 25 soldiers had been prescribed the drug, and only in cases of their intolerance for other alternatives. [ 38 ]  Between 2001 and 2012, 16,000 Canadian soldiers sent to Afghanistan were given the drug as a preventative measure. [ 38 ]  In 2013, the US Army banned mefloquine from use by its special forces such as the  Green Berets . [ 38 ]  In autumn 2016, the UK military followed suit with their Australian peers after a parliamentary inquiry into the matter revealed that it can cause permanent side effects and brain damage. [ 38 ]


In early December 2016, the German defence ministry removed mefloquine from the list of medications it would provide to its soldiers. [ 38 ]


In autumn 2016, Canadian  Surgeon General  Brigadier General  Hugh Colin MacKay  told a parliamentary committee that faulty science supported the assertion that the drug has indelible noxious side effects. An expert from  Health Canada  named  Barbara Raymond  told the same committee that the evidence she had read failed to support the conclusion of indelible side effects. [ 38 ]  Canadian soldiers who took mefloquine when deployed overseas have claimed they have been left with ongoing mental health problems. [ 40 ]


In 2020 the UK Ministry of Defence (MoD) admitted to a breach of duty regarding the use of Mefloquine. [ 41 ]  by acknowledging numerous instances of failure to assess the risks and warn of potential side effects of the drug.


In June 2010, the first case report appeared of a  progressive multifocal leukoencephalopathy  being successfully treated with mefloquine. Mefloquine can also act against the  JC virus . Administration of mefloquine seemed to eliminate the virus from the patient's body and prevented further neurological deterioration. [ 42 ]


Mefloquine alters  cholinergic   synaptic transmission  through both postsynaptic  [ 43 ]  and presynaptic actions. [ 44 ]  The postsynaptic action to inhibit  acetylcholinesterase  changes transmission across synapses in the brain. [ 45 ]


Metachirality  is a stronger form of  chirality .
It applies to objects or systems that are chiral (not identical to their  mirror image ) and where, in addition, their mirror image has a  symmetry group  that differs from the symmetry group of the original object or system. [ 1 ]


Many familiar chiral objects, like the capital letter 'Z' embedded in the plane, are not metachiral.
The symmetry group of the capital letter 'Z' embedded in the plane consists of the  identity transformation  and a rotation over 180˚ (a half turn).
In this case, the mirror image has the same symmetry group.
In particular, asymmetric objects (that only have the identity transformation as symmetry, like a human hand) are not metachiral,
since the mirror image is also asymmetric.
In general, two-dimensional objects and bounded three-dimensional objects are not metachiral.


An example of a metachiral object is an infinite  helical staircase .
A  helix  in 3D has a  handedness  (either left or right, like  screw thread ), whereby it differs from its mirror image.
An infinite helical staircase, however, does have symmetries:
 screw operations , that is, a combination of a  translation  and a  rotation .
The symmetry group of the mirror image of an infinite helical staircase also contains screw operations.
But they are of the opposite handedness and, hence,
the symmetry groups differ.
Note, however, that these symmetry groups are  isomorphic .


Of the 219  space groups , 11 are metachiral.
A nice example of a metachiral spatial structure is the  K 4  crystal , [ 2 ]  also known as Triamond,
and featured in the  Bamboozle  mathematical artwork. [ 3 ]


A  mirror image  (in a plane mirror) is a  reflected  duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface. As an  optical effect , it results from  specular reflection  off from surfaces of lustrous materials, especially a  mirror  or  water . It is also a concept in  geometry  and can be used as a  conceptualization  process for 3D structures.


In  geometry , the mirror image of an object or  two-dimensional figure  is the  virtual image  formed by  reflection  in a  plane mirror ; it is of the same size as the original object, yet different, unless the object or figure has  reflection symmetry  (also known as a  P-symmetry ).


Two-dimensional mirror images can be seen in the reflections of mirrors or other reflecting surfaces, or on a printed surface seen inside-out. If we first look at an object that is effectively two-dimensional (such as the writing on a card) and then turn the card to face a mirror, the object turns through an angle of 180° and we see a left-right reversal in the mirror. In this example, it is the change in orientation rather than the mirror itself that causes the observed reversal. Another example is when we stand with our backs to the mirror and face an object that is in front of the mirror. Then we compare the object with its reflection by turning ourselves 180°, towards the mirror. Again we perceive a left-right reversal due to a change in our orientation. So, in these examples the mirror does not actually cause the observed reversals.


The concept of reflection can be extended to  three-dimensional  objects, including the inside parts, even if they are not  transparent . The term then relates to structural as well as visual aspects. A three-dimensional object is reversed in the direction perpendicular to the mirror surface. In physics, mirror images are investigated in the subject called  geometrical optics . More fundamentally in geometry and mathematics they form the principal objects of  Coxeter group  theory and  reflection groups .


In chemistry, two versions ( isomers ) of a molecule, one a "mirror image" of the other, are called  enantiomers  if they are not "superposable" (the correct technical term, though the term "superimposable" is also used) on each other. That is an example of  chirality . In general, an object and its mirror image are called  enantiomorphs .


If a point of an object has coordinates ( x ,  y ,  z ) then the image of this point (as reflected by a mirror in the  y ,  z  plane) has coordinates (− x ,  y ,  z ). Thus reflection is a reversal of the coordinate axis perpendicular ( normal ) to the mirror's surface. Although a plane mirror reverses an object only in the direction normal to the mirror surface, this turns the entire three-dimensional image seen in the mirror inside-out, so there is a  perception  of a left-right reversal. Hence, the reversal is somewhat misleadingly called a "lateral inversion". The perception of a left-right reversal is geometrically explained by the fact that a three-dimensional object seen in a mirror is an inside-out version of the actual object, like a glove stripped off the left hand and turned into a right-hand glove, but there is still some confusion about the explanation amongst psychologists. The psychology of the perceived left-right reversal is discussed in "Much ado about mirrors" by Professor  Michael Corballis  (see "external links", below).


Reflection in a mirror  does  result in a change in  chirality , more specifically from a right-handed to a left-handed coordinate system (or vice versa). If one looks in a mirror two axes (up-down and left-right) coincide with those in the mirror, but  the third axis (front-back) is reversed.


If a person stands side-on to a mirror, left and right hands will be reversed  directly  by the mirror, because the person's left-right axis is then normal to the mirror plane. However, it is important to understand that there are  always  only two enantiomorphs, the object and its inside-out image. Therefore, no matter how the object is oriented towards the mirror, all the resulting images are fundamentally identical (as Corballis explains in his paper "Much ado about mirrors", mentioned above).


In the picture of the mountain reflected in the lake (photograph top right), the reversal normal to the reflecting surface is obvious. Notice that there is no obvious front-back or left-right of the mountain. In the example of the urn and mirror (photograph to right), the urn is fairly symmetrical front-back (and left-right). Thus, no obvious reversal of any sort can be seen in the mirror image of the urn.


A mirror image appears more obviously three-dimensional if the observer moves, or if the image is viewed using  binocular vision . This is because the relative position of objects changes as the observer's perspective changes, or is differently viewed with each eye. [ 1 ]


Looking through a mirror from different positions (but necessarily with the point of observation restricted to the halfspace on one side of the mirror) is like looking at the 3D mirror image of space; without further mirrors only the mirror image of the halfspace before the mirror is relevant; if there is another mirror, the mirror image of the other halfspace is too.


A mirror does not just produce an image of what would be there without it; it also changes the light distribution in the halfspace in front of and behind the mirror. A mirror hanging on the wall makes the room brighter because additional light sources appear in the mirror image. However, the appearance of additional light does not violate the  conservation of energy  principle, because some light no longer reaches behind the mirror, as the mirror simply re-directs the light energy. In terms of the light distribution, the virtual mirror image has the same appearance and the same effect as a real, symmetrically arranged half-space behind a window (instead of the mirror). Shadows may extend from the mirror into the halfspace before it, and vice versa.


In  mirror writing  a text is deliberately displayed as its mirror image, in order to be read through a mirror. For example, emergency vehicles such as  ambulances  or fire engines use mirror images in order to be read from a vehicle's  rear-view mirror . Some  movie theaters  also use mirror writing in a  Rear Window Captioning System  used to assist individuals with  hearing impairments  in watching films.


In the case of two mirrors, in planes at an angle α, looking through both from the sector which is the intersection of the two halfspaces, is like looking at a version of the world rotated by an angle of 2α; the points of observations and directions of looking for which this applies correspond to those for looking through a frame like that of the first mirror, and a frame at the mirror image with respect to the first plane, of the second mirror. If the mirrors have vertical edges then the left edge of the field of view is the plane through the right edge of the first mirror and the edge of the second mirror which is on the right when looked at directly, but on the left in the mirror image.


In the case of two parallel mirrors, looking through both at once is like looking at a version of the world which is translated by twice the distance between the mirrors, in the direction perpendicular to them, away from the observer. Since the plane of the mirror in which one looks directly is beyond that of the other mirror, one always looks at an oblique angle, and the translation just mentioned has not only a component away from the observer, but also one in a perpendicular direction. The translated view can also be described by a translation of the observer in opposite direction. For example, with a vertical  periscope , the shift of the world is away from the observer and down, both by the length of the periscope, but it is more practical to consider the equivalent shift of the observer: up, and backward.


It is also possible to create a  non-reversing mirror  by placing two  first surface mirrors  at 90º to give an image which is not reversed.


Mirror life  (also called  mirror-image life ) is a hypothetical form of life with  mirror-reflected  molecular building blocks. [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ]  The possibility of mirror life was first discussed by  Louis Pasteur . [ 6 ]  This alternative life form has never been discovered in nature, but efforts to build a mirror-image version of biology's molecular machinery are underway. [ 7 ]


In December 2024, a broad coalition of scientists, including leading synthetic biology researchers and Nobel laureates, warned that the creation of mirror life, including mirror bacteria, could cause "unprecedented and irreversible harm" to human health and ecosystems worldwide. [ 8 ] [ 9 ]  Its potential to escape immune defenses and invade natural ecosystems might lead to "pervasive lethal infections in a substantial fraction of plant and animal species, including humans." Given these risks, the scientists concluded that mirror organisms should not be created without compelling evidence of safety. [ 8 ]


Many of the essential molecules for life on Earth can exist in two mirror-image forms, often called "left-handed" and "right-handed", where handedness refers to the direction in which polarized light skews when beamed through a pure solution of the molecule, but living organisms do not use both. [ 10 ]   RNA  and  DNA  contain only right-handed  sugars ;  proteins  are exclusively composed of left-handed  amino acids , although many bacteria and fungi are able to synthesise non-ribosomal peptides containing right-handed amino acids, as the example of  peptidoglycan synthesis  shows. This phenomenon is known as  homochirality . [ 11 ]  It is not known whether homochirality emerged before or after life, whether the building blocks of life must have this particular chirality, or indeed whether life needs to be homochiral. [ 12 ]  Protein chains built from amino acids of mixed chirality tend not to fold or function well, but mirror-image proteins have been constructed that have identical function but on substrates of opposite handedness. [ 11 ]


Advances in  synthetic biology , like  synthesizing viruses  since 2002,  partially synthetic bacteria  in 2010, and  synthetic ribosomes  in 2013, may lead to the possibility of fully synthesizing a living cell from small molecules, which could enable synthesizing mirror cells from mirrored versions ( enantiomers ) of life's building-block molecules. Some proteins have been synthesized in mirror-image versions, including  polymerase  in 2016. [ 13 ] [ 14 ]


Reconstructing regular lifeforms in mirror-image form, using the mirror-image (chiral) reflection of their cellular components, could be achieved by substituting left-handed amino acids with right-handed ones, in order to create mirror reflections of proteins, and likewise substituting right-handed with left-handed nucleic acids. [ 15 ]  Because the  phospholipids  of cell membranes are also chiral, American geneticist  George Church  proposed using an achiral fatty acid instead of mirror-image phospholipids for the membrane. [ 15 ]


Electromagnetic force (chemistry) is unchanged under such molecular reflection transformation ( P-symmetry ). There is a small alteration of weak interactions under reflection, which can produce very small corrections that theoretically favor the natural enantiomers of amino acids and sugars, [ 16 ]  but it is unknown if this effect is large enough to affect the functionality of mirror biomolecules or explain homochirality in nature. [ 17 ]


Mirror animals would need to feed on reflected food, produced by reflected plants. Mirror viruses would not be able to attack natural cells, just as natural viruses would not be able to attack mirror cells. [ 15 ]


Mirror life presents potential dangers. For example, a chiral-mirror version of  cyanobacteria , which only needs achiral nutrients and light for  photosynthesis , could take over Earth's ecosystem due to lack of natural enemies, disturbing the bottom of the food chain by producing mirror versions of the required sugars. [ 15 ]  Some bacteria can digest  L -Glucose ; exceptions like this would give some rare lifeforms an unanticipated advantage.


Direct application of mirror-chiral organisms can be mass production of  enantiomers  (mirror-image) of molecules produced by normal life.


The creation of a mirror human is the basis of the 1950 short story " Technical Error " by Arthur C. Clarke. [ 21 ]  In this story, a physical accident transforms a person into his mirror image, speculatively explained by travel through a fourth physical dimension. H. G. Wells'  The Plattner Story  (1896) is based on a similar idea.


In the 1970  Star Trek  novel  Spock Must Die!  by James Blish, the science officer of the USS Enterprise is replicated in mirror form by a transporter mishap. He locks himself in the sick bay where he is able to synthesize mirror forms of basic nutrients needed for his survival. [ 22 ]


An alien machine that reverses chirality, and a blood-symbiont that functions properly only when in one chirality, were central to Roger Zelazny's 1976 novel  Doorways in the Sand . [ 23 ]


On the titular planet of  Sheri S. Tepper 's 1989 novel  Grass , some lifeforms have evolved to use the right-handed isomer of  alanine . [ 24 ]


In the  Mass Effect  series, chirality of amino acids in foodstuffs is discussed often in both dialogue and encyclopedia files.


In the 2014 science fiction novel  Cibola Burn  by  James S. A. Corey , the planet Ilus has indigenous life with partially-mirrored chirality. This renders human colonists unable to digest native flora and fauna, and greatly complicates conventional farming. Consequently, the colonists have to rely upon  hydroponic farming  and food importation. [ 25 ]


In the 2017  Daniel Suarez  novel  Change Agent , an antagonist, Otto, nicknamed the "Mirror Man", is revealed to be a genetically-engineered mirror human. He views other humans with disdain and causes them to feel an inexplicable repulsion by his very presence. [ 26 ]


The concept is used during  Ryan North 's 2023 run on  Fantastic Four  as an existential threat towards the human population. [ 27 ]


Planar chirality , also known as 2D chirality, is the special case of  chirality  for two  dimensions .


Most fundamentally, planar  chirality  is a  mathematical  term, finding use in  chemistry ,  physics  and related physical sciences, for example, in  astronomy ,  optics  and  metamaterials . Recent occurrences in latter two fields are dominated by  microwave  and  terahertz  applications as well as micro- and  nanostructured  planar interfaces for  infrared  and visible  light .


This term is used in  chemistry  contexts, [ 2 ]  e.g., for a  chiral   molecule  lacking an  asymmetric carbon  atom, but possessing two non- coplanar  rings that are each dissymmetric and which cannot easily rotate about the  chemical bond  connecting them: 2,2'-dimethylbiphenyl is perhaps the simplest example of this case. Planar chirality is also exhibited by molecules like ( E )- cyclooctene , some di- or poly-substituted  metallocenes , and certain monosubstituted  paracyclophanes . Nature rarely provides planar chiral molecules,  cavicularin  being an exception.


To assign the configuration of a planar chiral molecule, begin by selecting the pilot atom, which is the  highest priority  of the atoms that is not in the plane, but is directly attached to an atom in the plane. Next, assign the priority of the three adjacent in-plane atoms, starting with the atom attached to the pilot atom as priority 1, and preferentially assigning in order of highest priority if there is a choice. Then set the pilot atom to in front of the three atoms in question. If the three atoms reside in a clockwise direction when followed in order of priority, the molecule is assigned as R; when counterclockwise it is assigned as S. [ 3 ]


Papakostas  et al.  observed in 2003 that planar chirality affects the polarization of light diffracted by arrays of planar chiral microstructures, where large polarization changes of opposite sign were detected in light diffracted from planar structures of opposite handedness. [ 4 ]


The study of planar chiral  metamaterials  has revealed that planar chirality is also associated with an optical effect in non-diffracting structures: the directionally asymmetric transmission (reflection and absorption) of  circularly polarized  waves. Planar chiral metamaterials, which are also anisotropic and lossy exhibit different total transmission (reflection and absorption) levels for the same circularly polarized wave incident on their front and back.
The asymmetric transmission phenomenon arises from different, e.g. left-to-right, circular polarization conversion efficiencies for opposite propagation directions of the incident wave and therefore the effect is referred to as circular conversion dichroism.
Like the twist of a planar chiral pattern appears reversed for opposite directions of observation, planar chiral metamaterials have interchanged properties for left-handed and right-handed circularly polarized waves that are incident on their front and back. In particular left-handed and right-handed circularly polarized waves experience opposite directional transmission (reflection and absorption) asymmetries. [ 5 ] [ 6 ]


Achiral components may form a chiral arrangement. In this case, chirality is not an intrinsic property of the components, but rather imposed extrinsically by their relative positions and orientations. This concept is typically applied to experimental arrangements, for example, an achiral (meta)material illuminated by a beam of light, where the illumination direction makes the whole experiment different from its mirror image. Extrinsic planar chirality results from illumination of any periodically structured interface for suitable illumination directions. Starting from normal incidence onto a periodically structured interface, extrinsic planar chirality arises from tilting the interface around any axis that does not coincide with a line of mirror symmetry of the interface. In the presence of losses, extrinsic planar chirality can result in circular conversion dichroism, as described above. [ 7 ]


Conventional mirrors reverse the handedness of circularly polarized waves upon reflection. In contrast, a chiral mirror reflects circularly polarized waves of one handedness without handedness change [ dubious  –  discuss ] , while absorbing circularly polarized waves of the opposite handedness. A perfect chiral mirror exhibits circular conversion dichroism with ideal efficiency. Chiral mirrors can be realized by placing a planar chiral metamaterial in front of a conventional mirror. [ 8 ]  The concept has been exploited in holography to realize independent holograms for left-handed and right-handed circularly polarized electromagnetic waves. [ 9 ]  Active chiral mirrors that can be switched between left and right, or chiral mirror and conventional mirror, have been reported. [ 10 ]


Racemic acid  is an old name for an optically inactive or  racemic  form of  tartaric acid . It is an equal mixture of two mirror-image  isomers  ( enantiomers ), optically active in opposing directions. Racemic acid does not occur naturally in grape juice, although L-tartaric acid does.


Tartaric acid's sodium-ammonium  salt  is unusual among racemic mixtures in that during crystallization it can separate out into two kinds of crystals, each composed of one isomer, and whose macroscopic crystalline shapes are mirror images of each other. Thus,  Louis Pasteur  was able in 1848 to isolate each of the two enantiomers by laboriously separating the two kinds crystals using delicate tweezers and a hand lens. [ 1 ]  Pasteur announced his intention to resolve racemic acid in:


while he presented his resolution of racemic acid into separate  optical  isomers in:


In the latter paper, Pasteur sketches from natural concrete reality  chiral polytopes  quite possibly for the first time. The optical property of tartaric acid was first observed in 1832 by  Jean Baptiste Biot , who observed its ability to rotate  polarized light . [ 4 ] [ 5 ]  It remains unknown whether  Arthur Cayley  or  Ludwig Schläfli , or other contemporary mathematicians who studied polytopes, knew of the French work.


In two modern-day re-enactments performed in Japan of the Pasteur experiment, [ 6 ] [ 7 ]  it was established that the preparation of crystals was not very reproducible. The crystals deformed, but they were large enough to inspect with the naked eye (microscope not required).


Sinistral  and  dextral , in some scientific fields, are the two types of  chirality  (" handedness ") or  relative direction . The terms are derived from the  Latin  words for "left" ( sinister ) and "right" ( dexter ). Other disciplines use different terms (such as  dextro-  and  laevo-rotary  in  chemistry , or  clockwise  and  anticlockwise  in  physics ) or simply use left and right (as in  anatomy ).


Relative direction and chirality are distinct concepts. Relative direction is from the point of view of the observer; a completely symmetric object has a left side and a right side, from the observer's point of view, if the top and bottom and direction of observation are defined. Chirality, however, is observer-independent: no matter how one looks at a right-hand  screw thread , it remains different from a left-hand screw thread. Therefore, a symmetric object has sinistral and dextral directions arbitrarily defined by the position of the observer, while an asymmetric object that shows chirality may have sinistral and dextral directions defined by characteristics of the object, regardless of the position of the observer.


Because the coiled  shells  of  gastropods  are asymmetric, they possess a quality called  chirality –the "handedness" of an asymmetric structure.


Over 90% [ 1 ]  of gastropod species have shells in which the direction of the coil is  dextral  (right-handed). A small minority of species and genera have shells in which the coils are almost always  sinistral  (left-handed). Very few species show an even mixture of dextral and sinistral individuals (for example,  Amphidromus perversus ). [ 2 ]


The most obvious characteristic of  flatfish , other than their flatness, is their asymmetric morphology: both eyes are on the same side of the head in the adult fish. In some families of flatfish, the eyes are always on the right side of the body ( dextral  or right-eyed flatfish), and in others, they are always on the left ( sinistral  or left-eyed flatfish). Primitive  spiny turbots  include equal numbers of right- and left-sided individuals, and are generally more symmetric than other families. [ 3 ]


In  geology , the terms  sinistral  and  dextral  refer to the horizontal component of the movement of blocks on either side of a  fault  or the sense of movement within a  shear zone . These are terms of relative direction, as the movement of the blocks is described relative to each other when viewed from above. Movement is sinistral (left-handed) if the block on the other side of the fault moves to the left, or if straddling the fault the left side moves toward the observer. Movement is dextral (right-handed) if the block on the other side of the fault moves to the right, or if straddling the fault the right side moves toward the observer. [ 4 ]


In  chemistry , the term  supramolecular chirality  is used to describe  supramolecular assemblies  that are non-superposable on their mirror images.


Chirality  in  supramolecular chemistry  implies the non-symmetric arrangement of molecular components in a non-covalent assembly. Chirality may arise in a  supramolecular   system if one of its component is chiral or if achiral components arrange in a non symmetrical way to produce a  supermolecule  that is chiral. [ 1 ]