Chorded keyboard
A keyset or chorded keyboard (also called a chorded keyset, chord keyboard or chording keyboard) is a computer input device that allows the user to enter characters or commands formed by pressing several keys together, like playing a "chord" on a piano. The large number of combinations available from a small number of keys allows text or commands to be entered with one hand, leaving the other hand free. A secondary advantage is that it can be built into a device (such as a pocket-sized computer or a bicycle handlebar) that is too small to contain a normal-sized keyboard.

A chorded keyboard minus the board, typically designed to be used while held in the hand, is called a keyer. Douglas Engelbart introduced the chorded keyset as a computer interface in 1968 at what is often called "The Mother of All Demos".

Principles of operation

Each key is mapped to a number and then can be mapped to a corresponding letter or command. By pressing two or more keys together the user can generate many combinations. In Engelbart's original mapping, he used five keys: 1, 2, 4, 8, 16. The keys were mapped as follows: a = 1, b = 2, c = 3, d = 4, and so on. If the user pressed keys 1 + 2 = 3 simultaneously, and then released the keys, the letter "c" appeared. Unlike pressing a chord on a piano, the chord is recognized only after all the keys or mouse buttons are released. Since Engelbart introduced the keyset, several different designs have been developed based on similar concepts.

As a crude example, each finger might control one key which corresponds to one bit in a byte, so that using seven keys and seven fingers, one could enter any character in the ASCII set—if the user could remember the binary codes. Due to the small number of keys required, chording is easily adapted from a desktop to mobile environment.

Practical devices generally use simpler chords for common characters (e.g., Baudot), or may have ways to make it easier to remember the chords (e.g., Microwriter), but the same principles apply. These portable devices first became popular with the wearable computer movement in the 1980s.

Thad Starner from Georgia Institute of Technology and others published numerous studies showing that two-handed chorded text entry was faster and yielded fewer errors than on a QWERTY keyboard. Currently stenotype machines hold the record for fastest word entry.  Many stenotype users can reach 300 words per minute. However, stenographers typically train for three years before reaching professional levels of speed and accuracy.

History
The earliest known chord keyboard was part of the "five-needle" telegraph operator station, designed by Wheatstone and Cooke in 1836, in which any two of the five needles could point left or right to indicate letters on a grid. It was designed to be used by untrained operators (who would determine which keys to press by looking at the grid), and was not used where trained telegraph operators were available.

The first widespread use of a chord keyboard was in the stenotype machine used by court reporters, which was invented in 1868 and is still in use. The output of the stenotype was originally a phonetic code that had to be transcribed later (usually by the same operator who produced the original output), rather than arbitrary text—automatic conversion software is now commonplace.

In 1874, the five-bit Baudot telegraph code and a matching 5-key chord keyboard was designed to be used with the operator forming the codes manually. The code is optimized for speed and low wear: chords were chosen so that the most common characters used the simplest chords. But telegraph operators were already using typewriters with QWERTY keyboards to "copy" received messages, and at the time it made more sense to build a typewriter that could generate the codes automatically, rather than making them learn to use a new input device.

 

Some early keypunch machines used a keyboard with 12 labeled keys to punch the correct holes in paper cards. The numbers 0 through 9 were represented by one punch; 26 letters were represented by combinations of two punches, and symbols were represented by combinations of two or three punches.

Braille (a writing system for the blind) uses either 6 or 8 tactile 'points' from which all letters and numbers are formed. When Louis Braille invented it, it was produced with a needle holing successively all needed points in a cardboard sheet.  In 1892, Frank Haven Hall, superintendent of the Illinois Institute for the Education of the Blind, created the Hall Braille Writer, which was like a typewriter with 6 keys, one for each dot in a braille cell. The Perkins Brailler, first manufactured in 1951, uses a 6-key chord keyboard (plus a spacebar) to produce braille output, and has been very successful as a mass market affordable product. Braille, like Baudot, uses a number symbol and a shift symbol, which may be repeated for shift lock, to fit numbers and upper case into the 63 codes that 6 bits offer.

After World War II, with the arrival of electronics for reading chords and looking in tables of "codes", the postal sorting offices started to research chordic solutions to be able to employ people other than trained and expensive typists. In 1954, an important concept was discovered: chordic production is easier to master when the production is done at the release of the keys instead of when they are pressed.

Researchers at IBM investigated chord keyboards for both typewriters and computer data entry as early as 1959, with the idea that it might be faster than touch-typing if some chords were used to enter whole words or parts of words. A 1975 design by IBM Fellow Nat Rochester had 14 keys that were dimpled on the edges as well as the top, so one finger could press two adjacent keys for additional combinations.  Their results were inconclusive, but research continued until at least 1978.

Doug Engelbart began experimenting with keysets to use with the mouse in the mid 1960s. In a famous 1968 demonstration, Engelbart introduced a computer human interface that included the QWERTY keyboard, a three button mouse, and a five key keyset. Engelbart used the keyset with his left hand and the mouse with his right to type text and enter commands. The mouse buttons marked selections and confirmed or aborted commands.

Users in Engelbart's Augmentation Research Center at SRI became proficient with the mouse and keyset. In the 1970s the funding Engelbart's group received from the Advanced Research Projects Agency (ARPA) was cut and many key members of Engelbart's team went to work for Xerox PARC where they continued to experiment with the mouse and keyset. Keychord sets were used at Xerox PARC in the early 1980s, along with mice, GUIs, on the Xerox Star and Alto workstations. A one button version of the mouse was incorporated into the Apple Macintosh but Steve Jobs decided against incorporating the chorded keyset.

In the early 1980s, Philips Research labs at Redhill, Surrey did a brief study into small, cheap keyboards for entering text on a telephone. One solution used a grid of hexagonal keys with symbols inscribed into dimples in the keys that were either in the center of a key, across the boundary of two keys, or at the joining of three keys. Pressing down on one of the dimples would cause either one, two or three of the hexagonal buttons to be depressed at the same time, forming a chord that would be unique to that symbol. With this arrangement, a nine button keyboard with three rows of three hexagonal buttons could be fitted onto a telephone and could produce up to 33 different symbols. By choosing widely separated keys, one could employ one dimple as a 'shift' key to allow both letters and numbers to be produced. With eleven keys in a 3/4/4 arrangement, 43 symbols could be arranged allowing for lowercase text, numbers and a modest number of punctuation symbols to be represented along with a 'shift' function for accessing uppercase letters. While this had the advantage of being usable by untrained users via 'hunt and peck' typing and requiring one less key switch than a conventional 12 button keypad, it had the disadvantage that some symbols required three times as much force to depress them as others which made it hard to achieve any speed with the device. That solution is still alive and proposed by Fastap and Unitap among others, and a commercial phone has been produced and promoted in Canada during 2006.

Standards
Historically, the baudot and braille keyboards were standardized to some extent, but they are unable to replicate the full character set of a modern keyboard. Braille comes closest, as it has been extended to eight bits.

The only proposed modern standard, GKOS (or Global Keyboard Open Standard) can support most characters and functions found on a computer keyboard but has had little commercial development. There is, however, a GKOS keyboard application available for iPhone since May 8, 2010, for Android since October 3, 2010 and for MeeGo Harmattan since October 27, 2011.

Stenography
Stenotype machines (sometimes used by Court Reporters) use a chording keyboard to represent sounds: on the standard keyboard, the 'U' represents the sound (and word) 'you', and the three-key trigraph 'K' 'A' 'T' represents the sound and word 'cat'. The stenotype keyboard is explicitly ordered 'K', on the left, is the starting sound. 'S' and 'T', which are common starting sounds and also common ending sounds, are available on both sides of the keyboard: 'TAT' is 3-key chord, using both T keys. .

Open-source designs

Four open-source keyer/keyset designs are available: The pickey, a PS/2 device based on the PIC microcontroller; the spiffchorder, a USB device based on the Atmel AVR family of microcontrollers; the FeatherChorder, a BLE chorder based on the Adafruit Feather, an all-in-one board incorporating an Arduino-compatible microcontroller; and the GKOS keypad driver for Linux as well as the Gkos library for the Atmel/Arduino open-source board.

Plover is a free, open-source, cross-platform program intended to bring real-time stenographic technology not just to stenographers, but also to hobbyists using anything from professional Stenotype machines to low-cost NKRO gaming keyboards. It is available for Linux, Microsoft Windows, and Apple Mac macOS.

Joy2chord is a chorded keyboard driver for Linux. With a configuration file, any joystick or gamepad can be turned into a chorded keyboard. This design philosophy was decided on to lower the cost of building devices, and in turn lower the entry barrier to becoming familiar with chorded keyboards. Macro keys, and multiple modes are also easily implemented with a user space driver.

Commercial devices

One minimal chordic keyboard example is Edgar Matias' Half-Qwerty keyboard described in patent  circa 1992 that produces the letters of the missing half when the user simultaneously presses the space bar along with the mirror key. INTERCHI '93 published a study by Matias, MacKenzie and Buxton showing that people who have already learned to touch-type can quickly recover 50 to 70% of their two-handed typing speed. The loss contributes to the speed discussion above. It is implemented on two popular mobile phones, each provided with software disambiguation, which allows users to avoid using the space-bar.

"Multiambic" keyers for use with wearable computers were invented in Canada in the 1970s. Multiambic keyers are similar to chording keyboards but without the board, in that the keys are grouped in a cluster for being handheld, rather than for sitting on a flat surface.

Chording keyboards are also used as portable but two handed input devices for the visually impaired (either combined with a refreshable braille display or vocal synthesis). Such keyboards use a minimum of seven keys, where each key corresponds to an individual braille point, except one key which is used as a spacebar. In some applications, the spacebar is used to produce additional chords which enable the user to issue editing commands, such as moving the cursor, or deleting words. Note that the number of points used in braille computing is not 6, but 8, as this allows the user, among other things, to distinguish between small and capital letters, as well as identify the position of the cursor. As a result, most newer chorded keyboards for braille input include at least nine keys

Touch screen chordic keyboards are available to smartphone users as an optional way of entering text. As the number of keys is low the button areas can be made bigger and easier to hit on the small screen. The most common letters do not necessarily require chording as is the case with the GKOS keyboard optimised layouts (Android app) where the twelve most frequent characters only require single keys.

Historical
The WriteHander, a 12-key chord keyboard from NewO Company, appeared in 1978 issues of ROM Magazine, an early microcomputer applications magazine.

Another early commercial model was the six-button Microwriter, designed by Cy Endfield and Chris Rainey, and first sold in 1980. Microwriting is the system of chord keying and is based on a set of mnemonics. It was designed only for right-handed use.

In 1982 the Octima 8 keys cord keyboard was presented by Ergoplic Kebords Ltd an Israeli Startup that was founded by Israeli researcher with intensive experience in Man Machine Interface design. The keyboard had 8 keys one for each finger and additional 3 keys that enabled the production of numbers, punctuations and control functions.  The keyboard was fully compatible with the IBM PC & AT keyboards and had an Apple IIe version as well.  Its key combinations were based on a mnemonic system that enabled fast and easy touch type learning.  Within a few hours the user could achieve a typing speed similar to hand writing speed.  The unique design also gave a relief from hand stress (Carpal Tunnel Syndrome) and allowed longer typing sessions than traditional keyboards.  It was multi-lingual supporting English, German, French and Hebrew.

The BAT is a 7-key hand-sized device from Infogrip, and has been sold since 1985. It provides one key for each finger and three for the thumb.  It is proposed for the hand which does not hold the mouse, in an exact continuation of Engelbart's vision.

See also
BAT keyboard
FrogPad
Keyer
Microwriter
Palantype
Stenotype
Velotype syllable-chord keyboard

References

Bardini, Thierry, Bootstrapping: Douglas Engelbart, Coevolution, and the Origins of Personal Computing (2000), Chapters 2 & 3, , 
Engelbart and English, "A Research Center for Augmenting Human Intellect", AFIPS Conf. Proc., Vol 33, 1968 Fall Joint Computer Conference, p395-410
Lockhead and Klemmer, An Evaluation of an 8-Key Word-Writing Typewriter, IBM Research Report RC-180, IBM Research Center, Yorktown Heights, NY, Nov 1959.
Rochester, Bequaert, and Sharp, "The Chord Keyboard", IEEE Computer, December 1978, p57-63
Seibel, "Data Entry Devices and Procedures", in Human Engineering Guide to Equipment Design, Van Cott and Kinkade (Eds), 1963

Computer keyboard types
Physical ergonomics