A keyer is a device for signaling by hand, by way of pressing one or more switches. Modern keyers typically have a large number of switches but not as many as a full-size keyboard; typically between four and fifty. A keyer differs from a keyboard in the sense that there is no "board"; the keys are arranged in a cluster. A keyer may take the form of a single telegraph key for keying Morse code. In this use, the term "to key" means to turn on and off a carrier wave, typically. For example, it is said that one "keys the transmitter" by interrupting some stage of amplification with a telegraph key.
Morse code was an early form of serial communication, which in modern times is usually automated. In a completely automated teleprinter system, the sender presses keys to send an ASCII data stream to a receiver, and computation alleviates the need for timing to be done by the human operator. In this way, much higher typing speeds are possible.
Iambic keyers became popular, in telegraphy, in the 1960s. In these, the "dot" and the "dash" are separate keys. In the 1970s, when this concept was introduced to inventor Steve Mann, it was mistakenly heard by him as "biambic" so he generalized the term to include various "polyambic"/"multiambic" keyers, such as a "pentambic" keyer (5 keys, one for each finger and one for the thumb), and "septambic" (3 thumb buttons on a handgrip), for use with a portable backpack-based computer system that he invented for photographic lightvectoring.
Such keyers, used with wearable computers, are typically one-handed grips. Unlike keyboards, the wearable computer keyer has no board upon which the switches are mounted. Additionally, by providing some other function (such as simultaneous grip of flash light source and keying) the keyer is effectively hands free, in the sense one would still be holding the light source anyway.
Others have developed chording keyboards that are intended to be used while seated (multiple keys mounted to a board rather than a portable grip). One type of these, the half-QWERTY  uses only minimal chording, requiring the space bar to be pressed down if the alternate hand is used. It is otherwise a standard QWERTY keyboard of full size. It (and many other innovations in keyboard controls) was designed to deal with hand disability - one-armed or -handed persons in particular.
- Intelligent Image Processing. John Wiley and Sons. 2001.
- Summary of keyers, commercially made keyers, other links, etc. (archived version)
- GKOS back-panel chording keyboard, an open standard for handheld devices
- Twiddler 2 one-handed chording keyboard/mouse
- EkaPad one-handed, 12-key, chording keyboard
- Data Egg one-handed chording keyboard for handheld devices
- "Body-coupled FingeRing" — Wireless wearable chorded keyer
- Yet another one-hand keyboard — Do-it-yourself, wearable, one-hand keyboard prototypes
- Wearable keyer suitable for QRP use (leg mount accessory)