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5,101	Estimation of monitor size by the distance between opposite corners does not take into account the display aspect ratio, so that for example a 16:9 21-inch widescreen display has less area, than a 21-inch 4:3 screen. The 4:3 screen has dimensions of 16.8 in × 12.6 in and an area 211 sq in , while the widescreen is 18.3 in × 10.3 in , 188 sq in .
5,102	Until about 2003, most computer monitors had a 4:3 aspect ratio and some had 5:4. Between 2003 and 2006, monitors with 16:9 and mostly 16:10 aspect ratios became commonly available, first in laptops and later also in standalone monitors. Reasons for this transition included productive uses such as the word processor display of two standard letter pages side by side, as well as CAD displays of large-size drawings and application menus at the same time. In 2008 16:10 became the most common sold aspect ratio for LCD monitors and the same year 16:10 was the mainstream standard for laptops and notebook computers.
5,103	In 2010, the computer industry started to move over from 16:10 to 16:9 because 16:9 was chosen to be the standard high-definition television display size, and because they were cheaper to manufacture.
5,104	In 2011, non-widescreen displays with 4:3 aspect ratios were only being manufactured in small quantities. According to Samsung, this was because the "Demand for the old 'Square monitors' has decreased rapidly over the last couple of years," and "I predict that by the end of 2011, production on all 4:3 or similar panels will be halted due to a lack of demand."
5,105	The resolution for computer monitors has increased over time. From 280 × 192 during the late 1970s, to 1024 × 768 during the late 1990s. Since 2009, the most commonly sold resolution for computer monitors is 1920 × 1080, shared with the 1080p of HDTV. Before 2013 mass market LCD monitors were limited to 2560 × 1600 at 30 in , excluding niche professional monitors. By 2015 most major display manufacturers had released 3840 × 2160 displays, and the first 7680 × 4320 monitors had begun shipping.
5,106	Every RGB monitor has its own color gamut, bounded in chromaticity by a color triangle. Some of these triangles are smaller than the sRGB triangle, some are larger. Colors are typically encoded by 8 bits per primary color. The RGB value represents red, but slightly different colors in different color spaces such as Adobe RGB and sRGB. Displaying sRGB-encoded data on wide-gamut devices can give an unrealistic result. The gamut is a property of the monitor; the image color space can be forwarded as Exif metadata in the picture. As long as the monitor gamut is wider than the color space gamut, correct display is possible, if the monitor is calibrated. A picture that uses colors that are outside the sRGB color space will display on an sRGB color space monitor with limitations. Still today, many monitors that can display the sRGB color space are not factory nor user-calibrated to display it correctly. Color management is needed both in electronic publishing and in desktop publishing targeted to print.
5,107	Most modern monitors will switch to a power-saving mode if no video-input signal is received. This allows modern operating systems to turn off a monitor after a specified period of inactivity. This also extends the monitor's service life. Some monitors will also switch themselves off after a time period on standby.
5,108	Most modern laptops provide a method of screen dimming after periods of inactivity or when the battery is in use. This extends battery life and reduces wear.
5,109	Most modern monitors have two different indicator light colors wherein if video-input signal was detected, the indicator light is green and when the monitor is in power-saving mode, the screen is black and the indicator light is orange. Some monitors have different indicator light colors and some monitors have blinking indicator light when in power-saving mode.
5,110	Many monitors have other accessories integrated. This places standard ports within easy reach and eliminates the need for another separate hub, camera, microphone, or set of speakers. These monitors have advanced microprocessors which contain codec information, Windows interface drivers and other small software which help in proper functioning of these functions.
5,111	Monitors that feature an aspect ratio greater than 2:1 .Monitors with an aspect ratio greater than 3:1 are marketed as super ultrawide monitors. These are typically massive curved screens intended to replace a multi-monitor deployment.
5,112	These monitors use touching of the screen as an input method. Items can be selected or moved with a finger, and finger gestures may be used to convey commands. The screen will need frequent cleaning due to image degradation from fingerprints.
5,113	Some displays, especially newer flat panel monitors, replace the traditional anti-glare matte finish with a glossy one. This increases color saturation and sharpness but reflections from lights and windows are more visible. Anti-reflective coatings are sometimes applied to help reduce reflections, although this only partly mitigates the problem.
5,114	Most often using nominally flat-panel display technology such as LCD or OLED, a concave rather than convex curve is imparted, reducing geometric distortion, especially in extremely large and wide seamless desktop monitors intended for close viewing range.
5,115	Newer monitors are able to display a different image for each eye, often with the help of special glasses and polarizers, giving the perception of depth. An autostereoscopic screen can generate 3D images without headgear.
5,116	Features for medical using or for outdoor placement.
5,117	Narrow viewing angle screens are used in some security-conscious applications.
5,118	Integrated screen calibration tools, screen hoods, signal transmitters; Protective screens.
5,119	A combination of a monitor with a graphics tablet. Such devices are typically unresponsive to touch without the use of one or more special tools' pressure. Newer models however are now able to detect touch from any pressure and often have the ability to detect tool tilt and rotation as well.
5,120	Touch and tablet sensors are often used on sample and hold displays such as LCDs to substitute for the light pen, which can only work on CRTs.
5,121	The option for using the display as a reference monitor; these calibration features can give an advanced color management control for take a near-perfect image.
5,122	Option for professional LCD monitors, inherent to OLED & CRT; professional feature with mainstream tendency.
5,123	Near to mainstream professional feature; advanced hardware driver for backlit modules with local zones of uniformity correction.
5,124	Computer monitors are provided with a variety of methods for mounting them depending on the application and environment.
5,125	Raw monitors are raw framed LCD monitors, to install a monitor on a not so common place, ie, on the car door or you need it in the trunk. It is usually paired with a power adapter to have a versatile monitor for home or commercial use.
5,126	A desktop monitor is typically provided with a stand from the manufacturer which lifts the monitor up to a more ergonomic viewing height. The stand may be attached to the monitor using a proprietary method or may use, or be adaptable to, a VESA mount. A VESA standard mount allows the monitor to be used with more after-market stands if the original stand is removed. Stands may be fixed or offer a variety of features such as height adjustment, horizontal swivel, and landscape or portrait screen orientation.
5,127	The Flat Display Mounting Interface , also known as VESA Mounting Interface Standard or colloquially as a VESA mount, is a family of standards defined by the Video Electronics Standards Association for mounting flat panel displays to stands or wall mounts. It is implemented on most modern flat-panel monitors and TVs.
5,128	For computer monitors, the VESA Mount typically consists of four threaded holes on the rear of the display that will mate with an adapter bracket.
5,129	Rack mount computer monitors are available in two styles and are intended to be mounted into a 19-inch rack:
5,130	A fixed rack mount monitor is mounted directly to the rack with the flat-panel or CRT visible at all times. The height of the unit is measured in rack units and 8U or 9U are most common to fit 17-inch or 19-inch screens. The front sides of the unit are provided with flanges to mount to the rack, providing appropriately spaced holes or slots for the rack mounting screws. A 19-inch diagonal screen is the largest size that will fit within the rails of a 19-inch rack. Larger flat-panels may be accommodated but are 'mount-on-rack' and extend forward of the rack. There are smaller display units, typically used in broadcast environments, which fit multiple smaller screens side by side into one rack mount.
5,131	A stowable rack mount monitor is 1U, 2U or 3U high and is mounted on rack slides allowing the display to be folded down and the unit slid into the rack for storage as a drawer. The flat display is visible only when pulled out of the rack and deployed. These units may include only a display or may be equipped with a keyboard creating a KVM . Most common are systems with a single LCD but there are systems providing two or three displays in a single rack mount system.
5,132	A panel mount computer monitor is intended for mounting into a flat surface with the front of the display unit protruding just slightly. They may also be mounted to the rear of the panel. A flange is provided around the screen, sides, top and bottom, to allow mounting. This contrasts with a rack mount display where the flanges are only on the sides. The flanges will be provided with holes for thru-bolts or may have studs welded to the rear surface to secure the unit in the hole in the panel. Often a gasket is provided to provide a water-tight seal to the panel and the front of the screen will be sealed to the back of the front panel to prevent water and dirt contamination.
5,133	An open frame monitor provides the display and enough supporting structure to hold associated electronics and to minimally support the display. Provision will be made for attaching the unit to some external structure for support and protection. Open frame monitors are intended to be built into some other piece of equipment providing its own case. An arcade video game would be a good example with the display mounted inside the cabinet. There is usually an open frame display inside all end-use displays with the end-use display simply providing an attractive protective enclosure. Some rack mount monitor manufacturers will purchase desktop displays, take them apart, and discard the outer plastic parts, keeping the inner open-frame display for inclusion into their product.
5,134	According to an NSA document leaked to Der Spiegel, the NSA sometimes swaps the monitor cables on targeted computers with a bugged monitor cable in order to allow the NSA to remotely see what is being displayed on the targeted computer monitor.
5,135	Van Eck phreaking is the process of remotely displaying the contents of a CRT or LCD by detecting its electromagnetic emissions. It is named after Dutch computer researcher Wim van Eck, who in 1985 published the first paper on it, including proof of concept. Phreaking more generally is the process of exploiting telephone networks.
5,136	A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's cathode-ray tube display.
5,137	It allows the user to point to displayed objects or draw on the screen in a similar way to a touchscreen but with greater positional accuracy. A light pen can work with any CRT-based display, but its ability to be used with LCDs was unclear .
5,138	A light pen detects changes in brightness of nearby screen pixels when scanned by cathode-ray tube electron beam and communicates the timing of this event to the computer. Since a CRT scans the entire screen one pixel at a time, the computer can keep track of the expected time of scanning various locations on screen by the beam and infer the pen's position from the latest time stamps.
5,139	The first light pen, at this time still called "light gun", was created around 1951–1955 as part of the Whirlwind I project at MIT, where it was used to select discrete symbols on the screen, and later at the SAGE project, where it was used for tactical real-time-control of a radar-networked airspace.
5,140	One of the first more widely deployed uses was in the Situation Display consoles of the AN/FSQ-7 for military airspace surveillance. This is not very surprising, given its relationship with the Whirlwind projects. See Semi-Automatic Ground Environment for more details.
5,141	During the 1960s, light pens were common on graphics terminals such as the IBM 2250 and were also available for the IBM 3270 text-only terminal.
5,142	Light pen usage was expanded in the early 1980s to music workstations such as the Fairlight CMI and personal computers such as the BBC Micro. IBM PC-compatible MDA , CGA, HGC and some EGA graphics cards also featured a connector compatible with a light pen, as did early Tandy 1000 computers, the Thomson MO5 computer family, the Amiga, Atari 8-bit, Commodore 8-bit, some MSX computers and Amstrad PCW home computers. For the MSX computers, Sanyo produced a light pen interface cartridge.
5,143	Because the user was required to hold their arm in front of the screen for long periods of time or to use a desk that tilts the monitor, the light pen fell out of use as a general-purpose input device. Light pen was also perceived as working well only on displays with low persistance, which tend to flicker.
5,144	With most trackballs, operators have to lift their finger, thumb or hand and reposition in on the ball to continue rolling, whereas a mouse would have to be lifted itself and re-positioned. Some trackballs have notably low friction, as well as being made of a dense material such as phenolic resin, so they can be spun to make them coast. The trackball's buttons may be in similar positions to those of a mouse, or configured to suit the user.
5,145	Large trackballs are common on CAD workstations for easy precision. Before the advent of the touchpad, small trackballs were common on portable computers where there may be no desk space on which to run a mouse. Some small "thumballs" are designed to clip onto the side of the keyboard and have integral buttons with the same function as mouse buttons.
5,146	The trackball was invented as part of a post-World War II-era radar plotting system named Comprehensive Display System by Ralph Benjamin when working for the British Royal Navy Scientific Service. Benjamin's project used analog computers to calculate the future position of target aircraft based on several initial input points provided by a user with a joystick. Benjamin felt that a more elegant input device was needed and invented a ball tracker system called the roller ball for this purpose in 1946. The device was patented in 1947, but only a prototype using a metal ball rolling on two rubber-coated wheels was ever built and the device was kept as a military secret. Production versions of the CDS used joysticks.
5,147	The CDS system had also been viewed by a number of engineers from Ferranti Canada, who returned to Canada and began development of the Royal Canadian Navy's DATAR system in 1952. Principal designers Tom Cranston, Fred Longstaff and Kenyon Taylor chose the trackball as the primary input, using a standard five-pin bowling ball as the roller. DATAR was similar in concept to Benjamin's display, but used a digital computer to calculate tracks, and sent the resulting data to other ships in a task force using pulse-code modulation radio signals.
5,148	DATAR's trackball used four disks to pick up motion, two each for the X and Y directions. Several additional rollers provided mechanical support. When the ball was rolled, the pickup discs spun and contacts on their outer rim made periodic contact with wires, producing pulses of output with each movement of the ball. By counting the pulses, the physical movement of the ball could be determined.
5,149	Since 1966, the American company Orbit Instrument Corporation produced a device named X-Y Ball Tracker, a trackball, which was embedded into radar flight control desks.
5,150	A similar trackball device at the German Bundesanstalt für Flugsicherung was constructed by a team around Rainer Mallebrein of Telefunken Konstanz as part of the development for the Telefunken computer infrastructure around the main frame TR 440 , process computer TR 86 and video terminal SIG 100-86, which began in 1965. This trackball was called Rollkugel . Somewhat later, the idea of "reversing" this device led to the introduction of the first computer ball mouse , which was offered as an alternative input device to light pens and trackballs for Telefunken's computer systems since 1968.
5,151	In later trackball models the electrical contacts were replaced by an optical chopper wheel, which had small slots cut into it in rather than electrical contacts. With an LED for illumination from one side and an optical sensor on the other, rotation of the wheel periodically blocks and unblocks the light, so the sensor produces electrical pulses to indicate that rotation is occurring.
5,152	Mice used the same basic system for determining motion, but had the problem that the ball was in contact with the desk or mousepad. In order to provide smooth motion the balls were often covered with an anti-slip surface treatment, which was, by design, sticky. Rolling the mouse tended to pick up any dirt and drag it into the system where it would clog the chopper wheels, demanding cleanup. In contrast the trackball is in contact only with the user's hand, which tends to be cleaner. In the late 1990s both mice and trackballs began using direct optical tracking which follows dots on the ball, avoiding the need for anti-slip surface treatment.
5,153	As with modern mice, most trackballs now have an auxiliary device primarily intended for scrolling. Some have a scroll wheel like most mice, but the most common type is a “scroll ring” which is spun around the ball. Kensington's SlimBlade Trackball similarly tracks the ball itself in three dimensions for scrolling.
5,154	As of 1989 and into the 2020s, two major companies developed and produce consumer trackballs, Logitech and Kensington, although Logitech has narrowed its product line to two models. Other smaller companies occasionally offer a trackball in their product line. Microsoft produced popular models including The Microsoft Trackball Explorer, but has since discontinued all of its products.
5,155	In September 2017 Logitech announced release of MX-Ergo Mouse, which was released after 6 years of its last trackball mouse.
5,156	Large trackballs are sometimes seen on computerized special-purpose workstations, such as the radar consoles in an air-traffic control room or sonar equipment on a ship or submarine. Modern installations of such equipment may use mice instead, since most people now already know how to use one. However, military mobile anti-aircraft radars, commercial airliners and submarine sonars tend to continue using trackballs, since they can be made more durable and more fit for fast emergency use. Large and well made ones allow easier high precision work, for which reason they may still be used in these applications and in computer-aided design.
5,157	Trackballs have appeared in video games, particularly early arcade games . In March 1978, Sega released World Cup, an association football game with trackball controls. In October 1978, Atari released Atari Football, which popularized the use of a trackball, with the game's developers mentioning it was inspired by an earlier Japanese association football game. Other notable trackball games include Atari's Centipede and Missile Command – Atari trademarked it "TRAK-BALL".
5,158	Console trackballs, now fairly rare, were common in the early 1980s: the Atari 2600 and 5200 consoles, as well as the competing ColecoVision console, though using a joystick as their standard controller, each had one as an optional peripheral. The Apple Pippin, a console introduced in 1996, had a trackball built into its gamepad as standard. Trackballs were occasionally used in e-sports prior to the mainstreaming of optical mice in the early 2000s because they were more reliable than ball mice, but now they are extremely rare because optical mice offer superior speed and precision. Trackballs remain in use in pub golf machines to simulate swinging the club.
5,159	Trackballs have also been regarded as excellent complements to analog joysticks, as pioneered by the Assassin 3D, a trackball released in 1996 with joystick pass-through capability. Later in 1996, Mad Catz released the Panther XL, which was based on the Assassin 3D. This combination provides for two-hand aiming and a high accuracy and consistency replacement for the traditional mouse and keyboard combo generally used on first-person shooter games. Many such games natively support joysticks and analog player movement, like Valve's Half-Life and id Software's Quake series. As of 2020, one professional eSport player was known for using a trackball.
5,160	Trackballs are provided as the pointing device in some public internet access terminals. Unlike a mouse, a trackball can easily be built into a console, and cannot be ripped away or easily vandalized. Two examples are the Internet browsing consoles provided in some UK McDonald's outlets, and the BT Broadband Internet public phone boxes. This simplicity and ruggedness also makes them ideal for use in industrial computers.
5,161	Because trackballs for personal computers are stationary, they may require less space for operation than a mouse, simplifying use in confined or cluttered areas such as a small desk or a rack-mounted terminal. They are generally preferred in laboratory setting for the same reason.
5,162	Trackballs were often included in laptop computers, but since the late 1990s these have been replaced by touchpads and pointing sticks. Trackballs are still used as separate input devices with standard desktop computers, but this application is also moving to touchpads due to the prevalence of multi touch gesture control in new desktop operating systems.
5,163	People with a mobility impairment use trackballs as an assistive technology input device. Access to an alternative pointing device has become even more important for them with the dominance of graphically-oriented operating systems. There are many alternative systems to be considered. The control surface of a trackball is easier to manipulate and the buttons can be activated without affecting the pointer position.
5,164	Trackball users also often state that they are not limited to using the device on a flat desk surface. Trackballs can be used whilst browsing a laptop in bed, or wirelessly from an armchair to a PC playing a movie. They are also useful for computing on boats or other unstable platforms where a rolling deck could produce undesirable input.
5,165	Trackballs are generally either thumb-operated, with a ball about an inch in diameter or smaller moved by one digit and the buttons clicked by others, or finger-operated, with a ball over two inches in diameter operated by the middle fingers and the buttons by the thumb and little finger. Users favor one format or another for reasons of comfort, mobility, precision, or because it reduces strain on one part of the hand/wrist. Most, but not all, finger-operated designs are symmetrical in design, making them usable by both hands, while thumb-operated designs are by their nature asymmetric or “handed,” allowing the smallest examples to be held in the air. Thumb-operated trackballs are not generally available in left-handed configurations, due to small demand.
5,166	Some computer users prefer a trackball over the more common mouse for ergonomic reasons. There seems to be no conclusive evidence from studies performed to determine which type of pointing device works best for most applications. Application users are encouraged to test different devices, and to maintain proper posture and scheduled breaks for comfort. Some disabled users find trackballs easier since they only have to move their thumb relative to their hand, instead of moving the whole hand, while others incur unacceptable fatigue of the thumb. Elderly people sometimes have difficulty holding a mouse still while double-clicking; the trackball allows them to let go of the ball while using the button.
5,167	At times when a user is browsing menus or websites rather than typing, it is also possible to hold a trackball in the right hand like a television remote control, operating the ball with the right thumb and pressing the buttons with the left thumb, thus giving the fingers a rest.
5,168	Some mobile devices have trackballs, including those in the BlackBerry range, the T-Mobile Sidekick 3, and many early HTC smartphones. These miniature trackballs are made to fit within the thickness of a mobile device, and are controlled by the tip of a finger or thumb. These have mostly been replaced on smartphones by touch screens, although on the BlackBerry range they were replaced by an "optical trackball" or "optical trackpad" before later being replaced with touch screens.
5,169	In lieu of a scroll wheel, some mice include a tiny trackball sometimes called a scroll ball. A popular example is Apple's Mighty Mouse. Mice with a larger trackball on a side may be designed to stay stationary, using the trackball to move the mouse cursor instead of moving the mouse.
5,170	Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time of day.
5,171	The term real-time clock is used to avoid confusion with ordinary hardware clocks which are only signals that govern digital electronics, and do not count time in human units. RTC should not be confused with real-time computing, which shares its three-letter acronym but does not directly relate to time of day.
5,172	Although keeping time can be done without an RTC, using one has benefits:
5,173	A GPS receiver can shorten its startup time by comparing the current time, according to its RTC, with the time at which it last had a valid signal. If it has been less than a few hours, then the previous ephemeris is still usable.
5,174	Some motherboards are made without RTCs. The RTC may be omitted out of desire to save money or reduce possible sources of hardware failure.
5,175	RTCs often have an alternate source of power, so they can continue to keep time while the primary source of power is off or unavailable. This alternate source of power is normally a lithium battery in older systems, but some newer systems use a supercapacitor, because they are rechargeable and can be soldered. The alternate power source can also supply power to battery backed RAM.
5,176	Most RTCs use a crystal oscillator, but some have the option of using the power line frequency. The crystal frequency is usually 32.768 kHz, the same frequency used in quartz clocks and watches. Being exactly 215 cycles per second, it is a convenient rate to use with simple binary counter circuits. The low frequency saves power, while remaining above human hearing range. The quartz tuning fork of these crystals does not change size much from temperature, so temperature does not change its frequency much.
5,177	Some RTCs use a micromechanical resonator on the silicon chip of the RTC. This reduces the size and cost of an RTC by reducing its parts count. Micromechanical resonators are much more sensitive to temperature than quartz resonators. So, these compensate for temperature changes using an electronic thermometer and electronic logic.
5,178	Typical crystal RTC accuracy specifications are from ±100 to ±20 parts per million , but temperature-compensated RTC ICs are available accurate to less than 5 parts per million. In practical terms, this is good enough to perform celestial navigation, the classic task of a chronometer. In 2011, chip-scale atomic clocks became available. Although vastly more expensive and power-hungry , they keep time within 50 parts per trillion .
5,179	Many integrated circuit manufacturers make RTCs, including Epson, Intersil, IDT, Maxim, NXP Semiconductors, Texas Instruments, STMicroelectronics and Ricoh. A common RTC used in single-board computers is the Maxim Integrated DS1307.
5,180	The RTC was introduced to PC compatibles by the IBM PC/AT in 1984, which used a Motorola MC146818 RTC. Later, Dallas Semiconductor made compatible RTCs, which were often used in older personal computers, and are easily found on motherboards because of their distinctive black battery cap and silkscreened logo. A standard CMOS interface is available for the PC RTC.
5,181	In newer computer systems, the RTC is integrated into the southbridge chip.
5,182	Some microcontrollers have a real-time clock built in, generally only the ones with many other features and peripherals.
5,183	Some modern computers receive clock information by digital radio and use it to promote time-standards. There are two common methods: Most cell phone protocols directly provide the current local time. If an internet radio is available, a computer may use the network time protocol. Computers used as local time servers occasionally use GPS or ultra-low frequency radio transmissions broadcast by a national standards organization .
5,184	The following system is well-known to embedded systems programmers, who sometimes must construct RTCs in systems that lack them. Most computers have one or more hardware timers that use timing signals from quartz crystals or ceramic resonators. These have inaccurate absolute timing that is yet very repeatable . Software can do the math to make these into accurate RTCs. The hardware timer can produce a periodic interrupt, e.g. 50 Hz, to mimic a historic RTC . However, it uses math to adjust the timing chain for accuracy:
5,185	time = time + rate.
5,186	When the "time" variable exceeds a constant, usually a power of two, the nominal, calculated clock time is subtracted from "time", and the clock's timing-chain software is invoked to count fractions of seconds, seconds, etc. With 32-bit variables for time and rate, the mathematical resolution of "rate" can exceed one part per billion. The clock remains accurate because it will occasionally skip a fraction of a second, or increment by two fractions. The tiny skip is imperceptible for almost all real uses of an RTC.
5,187	The complexity with this system is determining the instantaneous corrected value for the variable "rate". The simplest system tracks RTC time and reference time between two settings of the clock, and divides reference time by RTC time to find "rate". Internet time is often accurate to less than 20 milliseconds, so 8000 or more seconds of separation between settings can usually divide the forty milliseconds of error to less than 5 parts per million to get chronometer-like accuracy. The main complexity with this system is converting dates and times to counts of seconds, but methods are well known.
5,188	If the RTC runs when a unit is off, usually the RTC will run at two rates, one when the unit is on and another when off. This is because the temperature and power-supply voltage in each state is consistent. To adjust for these states, the software calculates two rates. First, software records the RTC time, reference time, on seconds and off seconds for the two intervals between the last three times that the clock is set. Using this, it can measure the accuracy of the two intervals, with each interval having a different distribution of on and off seconds. The rate math solves two linear equations to calculate two rates, one for on and the other for off.
5,189	Another approach measures the temperature of the oscillator with an electronic thermometer, and uses a polynomial to calculate "rate" about once per minute. These require a calibration that measures the frequency at several temperatures, and then a linear regression to find the equation of temperature. The most common quartz crystals in a system are SC-cut crystals, and their rates over temperature can be characterized with a 3rd-degree polynomial. So, to calibrate these, the frequency is measured at four temperatures. The common tuning-fork-style crystals used in watches and many RTC components have parabolic equations of temperature, and can be calibrated with only 3 measurements. MEMS oscillators vary, from 3rd degree to fifth degree polynomials, depending on their mechanical design, and so need from four to six calibration measurements. Something like this approach might be used in commercial RTC ICs, but the actual methods of efficient high-speed manufacturing are proprietary.
5,190	Some computer designs such as smaller IBM System/360s, PDP-8s and Novas used a real-time clock that was accurate, simple and low cost. In Europe, North America and some other grids, the frequency of the AC mains is adjusted to the long-term frequency accuracy of the national standards. In those grids, clocks using AC mains can keep perfect time without adjustment. Such clocks are not practical in portable computers or grids that do not regulate the frequency of AC mains.
5,191	These computers' power supplies use a transformer or resistor divider to produce a sine wave at logic voltages. This signal is conditioned by a zero crossing detector, either using a linear amplifier, or a schmitt trigger. The result is a square wave with single, fast edges at the mains frequency. This logic signal triggers an interrupt. The interrupt handler software usually counts cycles, seconds, etc. In this way, it can provide an entire clock and calendar. In the IBM 360, the interrupt updates a 64-bit count of microseconds utilized by standardized systems software. The clock's jitter error is half if the clock interrupts for each zero crossing, instead of each cycle.
5,192	The clock also usually formed the basis of computers' software timing chains; e.g. it was usually the timer used to switch tasks in an operating system. Counting timers used in modern computers provide similar features at lower precision, and may trace their requirements to this type of clock.
5,193	A software-based clock must be set each time its computer is turned on. Originally this was done by computer operators. When the Internet became commonplace, network time protocols were used to automatically set clocks of this type.
5,194	Overlay keyboards generally consist of a flat grid of unmarked buttons. A sheet called an overlay is placed on the keyboard to identify each key, after the keyboard is programmed. The overlay can consist of any combination of words, symbols, or pictures.
5,195	Overlay keyboards have several advantages over conventional keyboards or mice. They do not require memorization of shortcut keys nor do they require a great deal of fine motor control, making them ideal for people who have difficulty using a conventional keyboard. Overlay keyboards are easy to clean, and resistant to spills or dust. The ability to change overlay sheets also makes it easy for a single overlay keyboard to have several different uses.
5,196	Overlay keyboards are probably most often found in fast food restaurants, where they reduce the amount of time required to enter items. Overlay keyboards are also used in education, especially at the primary level. They can also be used by disabled people who have sensory or motor control difficulties.
5,197	A computer mouse is a hand-held pointing device that detects two-dimensional motion relative to a surface. This motion is typically translated into the motion of the pointer on a display, which allows a smooth control of the graphical user interface of a computer.
5,198	The first public demonstration of a mouse controlling a computer system was done by Doug Engelbart in 1968 as part of the Mother of All Demos. Mice originally used two separate wheels to directly track movement across a surface: one in the x-dimension and one in the Y. Later, the standard design shifted to use a ball rolling on a surface to detect motion, in turn connected to internal rollers. Most modern mice use optical movement detection with no moving parts. Though originally all mice were connected to a computer by a cable, many modern mice are cordless, relying on short-range radio communication with the connected system.
5,199	In addition to moving a cursor, computer mice have one or more buttons to allow operations such as the selection of a menu item on a display. Mice often also feature other elements, such as touch surfaces and scroll wheels, which enable additional control and dimensional input.
5,200	The earliest known written use of the term mouse or mice in reference to a computer pointing device is in Bill English's July 1965 publication, "Computer-Aided Display Control". This likely originated from its resemblance to the shape and size of a mouse, with the cord resembling its tail. The popularity of wireless mice without cords makes the resemblance less obvious.

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