The conventional music keyboard is still the primary method of playing and controlling most synths, although many other devices for controlling the playing of notes has been devised over the years. The conventional keyboard uses a layout of seven "natural" or white keys, and five "accidental" or black keys, per octave. The white keys are flat on top, butted up against each other (no or minimal gaps between keys), and extend out towards the player farther than the black keys. The black keys sit above the white keys; they are narrow enough that the "legs" of the white keys can extend back towards the key hinge and operating mechanism in between the black keys. The black keys usually have their front faces (the edge facing the player) cut at an angle; the tops may be flat or contoured from front to rear. All key edges are blunted and rounded off.
The music keyboard originated with the hydraulic pipe organs of ancient Rome. These were played via wooden levers that extended out from a vertical panel; because the mechanism was crude and heavy, the keyboard was played by hitting the levers with the fist. In the Middle Ages, pipe organ builders started to work out better mechanisms and the music keyboard began to emerge. The number and arrangement of the keys varied significantly; the modern equal tempered scale had not been worked out at the time and not all of the accidental notes had been identified, which limited the chords and scales that could be played. Around Beethoven's time, the now-standard 12-note chromatic scale was worked out and the basic arrangement of keys standardized.
Plucked and struck stringed instruments including the clavichord and harpsichord adopted the keyboard as their performer interface during the Baroque era, which led to some variations in performer control. The pipe organs had always used a "tracker" mechanism by which pressing the key, through an arrangement of levers, snapped the pipe's pallet valves open and shut; little expression control was possible. Some of the new string instruments had some expression capability, such as the aftertouch-like "bebung" vibrato possible with the clavichord. Of course, the piano later arrived with significant expression control built into the key mechanism, as well as a significantly different key feel from the pipe organ. These concepts woudl influence later synth keyboard development.
At the beginning of the 20th century, the first "electric action" organ consoles were developed. This replaced the track levers attached to each key with an electrical swtich under the key; this delivered an electrical signal to a solenoid-controlled pallet valve. Since the traditional tracker mechanism provided little expression, builders found that they could use a simple on-off switch under the key and there was no loss of playability. In fact, playability often improved since it was possible to make the feel of the keys and switches lighter and more consistent than what was possible with the tracker mechanism. Some builders did try to provide some expression capability; some Wurlitzer organ consoles contained a capability called "second touch", consisting of a second switch under each key which was activated by pressing the key harder. The second touch could be used to trigger an additional function, such as bringing in another rank of pipes on the note, or adding a percussion instrument to the note attack.
The next step was electric organs. These used the same basic mechanism of switches under the keys, although in some cases they were more complex. The Hammond organs used a set of nine bus bars and nine contacts connected to each key, in order to eliminate the entirely separate mechanism for "stop" control that pipe organs required. Again, these were simple on-off switches, which sufficed since there was no expectation for expression control through the keys; electric organs, like most pipe organs, used a separate pedal for dynamics control.
The use of the keyboard with synthesizers was a sort of historical accident, stemming from the availability of keyboard mechanisms by scrapping old electric organs. In th earliest days of the synthesizer, there was no fixed idea of how the performer should control the instrument; many early Buchla synths offered an array of capacitive touch pads, and other mechanisms such as joysticks were common. Moog Music, seeking a control mechanism that would be familiar to performers of conventional instruments, realized that an organ keyboard could be wired via a "ladder" of resistors to produce a control voltage that would be proportional to the key played, providing a control signal that could be used to control the frequency of a VCO, and with proper scaling the result would be to play the same note that would be played if the same key were pressed on a piano or organ. The on/off nature of the key switches produced a simple gate signal, which Moog reasoned would be adequate since dynamics could be created by way of an envelope generator triggered by the gate signal.
This was fine for monophonic synths, and through clever wiring it was possible to produce control voltages for two notes at once, but in order to press polyphony further, something else was needed. The answer arrived in the 1970s in the form of the scanning keyboard, in which a microprocessor rapidly reads the on-off state of each key and produces, by means of a D/A converter, a control voltage for each key that is pressed. This was also the key to bringing in expression control, in the form of velocity sensing. In a velocity-sensitive keyboard, each key has two switch contacts, one that closes when the key is partly down, and a second that closes when the key is fully down. By having the microprocessor measure the (very short) time between the two switch closures, it could figure out how rapidly the performer pressed the key and produce a proportional control signal, in effect duplicating what the piano key mechanism does mechanically. Additionally, aftertouch could be provided buy having the microprocessor read the value of a force-sensitive resistor mounted underneath the key.
Synth keyboards vary in terms of their span (number of keys). Keyboards that are built into synths are commonly 37 keys (three octaves) or 49 keys (four octaves) with C keys on both ends, although many Moog synths have keyboards with F as the lowest note. However, controller keyboards are available ranging from 2 to 8 octaves in span.
Most keyboards use standard-sized keys, based on the dimensions of the modern piano keyboard. "Mini" keyboards have keys about 2/3 the size, in all dimensions, of a standard keyboard. They save space and make it easier for the performer to play large intervals with one hand, but can be awkward for players with larger hands and fingers. They also tend to have short key throws, which creates a different "feel" to the keyboard.
The way that the front edge (the edge facing the performer) of the white keys are shape and protected varies. "Waterfall" keys, as seen on the Hammond organs, have a square face to the player; more common is a thin projecting edge which makes playing glissandos a bit less rough on the hands. Some synths have these projecting edges sticking out past the edge of the case, which makes the keys vulnerable to damage in transit.