What are piano keys?
Piano keys are the black and white bars on a piano keyboard. Black keys are called so because they were originally covered with wax to hide their "unpleasant" appearance. The term sharps or flats refer to how that pitch should be played in comparison to another pitch, not whether it is a white key or a black key.
The number of keys on a piano varies depending on the model of the piano being used. Pianos can have as few as 61 keys (for smaller electronic pianos) and much larger 88-key models (including both full-size grand pianos and upright/vertical stringed ones).
The modern piano grew out of improvements made to earlier models over many years. First called a fortepiano ("loud-soft" or "strong-weak"), its lineal ancestor came from Italy around 1700 and was known as an arpicembalo ("harp-harpsichord").
The next major step was the bentside spinet, which used a harpsichord's "plucking" action but had a small piano-type hammer instead of jacks. These were followed by the square piano with its diagonal shape and horizontal strings, then by smaller uprights that evolved into modern grands.
Keyboard instruments
Keyboard instruments are broadly divided into two types, electronic keyboards, and acoustic pianos. Most modern electronic keyboards are MIDI controllers that can be connected to a synthesizer or other keyboard amp for speaker systems.
The number of keys on an electric piano varies. Some models contain the same number as most standard pianos (88-keys). Other models have fewer keys, such as three octaves (typically 35 keys). Much larger electric pianos with more keys exist, though their feed mechanism is usually not touch-sensitive.
Determining how many keys to include in a digital keyboard depends on the technology used and its purpose: portable digital keyboards typically use 44 miniaturized low-powered velocity-sensitive keys; consumer digital pianos often contain synthetic ivory-feel keys, and MIDI controller keyboards include a variety of control options to suit the player's preference.
Steinway & Sons grand pianos have a total range of just over six octaves, from C to F8 (C3 –F6, 7 Octaves). The size of the low bass strings on a piano determines the lowest note's fundamental frequency, which establishes the upper limit for determining how many keys can be accommodated within an 88 key span.
Notes below this range are called pedal tones or pedals. Pedal tone frequencies calculated in hertz with no consideration given to how fast the key reaches its destination will produce longer values than those expected by considering how quickly a key travels from rest to its fully depressed position.
High-speed key down and up measurements combined with low-frequency information can produce notes below the lowest key on a piano.
Frequency is not determined solely by the length of a string; it also depends upon how heavily the string is struck (known as "force" or, in musical terms, "pressure"). A lightly played note could be amplified enough to exceed the standard pitch range. In practice, there is no such thing as one universal standard for determining which frequencies should be considered pedal tones; different manufacturers make different assumptions about how quickly a key moves from rest to its fully depressed position when designing their pianos.
88 keyboard for pianos
The number of keys on a piano is standardized in the MIDI specification at 88, although this does not refer to the standard range played by an average human hand. The number of keys on pianos has been standardized at 88 since the late 19th century. A variety of sounds can be made by striking strings with soon or hammers, plucking them with the fingers, or rubbing their strings with various implements.
The 88 key limitation was chosen due to limitations in mechanical linkage design. If there were fewer keys, say 74, then that would allow for 14 more multi-note combinations available in the rightmost four octaves. These 14 multi-note combinations were deemed impractical because it requires three and a half extra turns from when the key is pressed until when all required string(s) are allowed to sound (the note of time).
As such, 88 keys allow direct access to all possible notes within this three-and-a-half octave range or five complete octaves without requiring additional key presses.
Pianos with their full range of 88 keys are large compared to most other keyboard instruments, such as electric or electronic keyboards, harpsichords, clavichords, and synthesizers. Consider that a full-size grand piano is four and a half feet long, whereas other keyboards vary from less than two feet (a typical 88-key electronic keyboard) to 24 feet (the Bösendorfer 290 Imperial Grand).
Other piano key keyboards
Some keys are provided for specific functions, such as changing the sustain or expression using the damper pedal. On some keyboards, these extended "function" keys are programmable to load other instrument sounds instead of piano tones.
Many are also adjustable to control keyboard response (the speed at which each key reacts), enabling a pianist to match an appropriate key action to his preference. 97 key keyboards for pianos are not standardized in the MIDI specification.
The next most common range is C2 to F6 (30 keys), which provides extended coverage of the higher octaves. On electronic keyboards with 61 or 76 notes, this can be obtained by combining two instruments that cover these ranges into one keyboard. Alternatively, some manufacturers sell less costly 61-key or 76-key models designed to fit within an ensemble's existing setup without compromising quality or performance.
Some smaller pianos are built with 36, 32, 30, 28, 25, or 23 notes. Some of these are rarer than others. 32-note pianos are only found in 6 to 7 foot upright pianos, which are too small for serious performance. 30-note pianos were available in the mid 20th century but became obsolete when players began to use larger pianos or electronic keyboards with piano samples instead of smaller instruments that struggled to project sufficient sound.
Some 25-note spinets do still exist, but these are even rarer because they only cover five octaves and therefore have diminished value in today's musical marketplace. The smallest pianos, such as the 22-note mini toy piano or the 23-note spinet, are not true pianos at all.
Pianos with fewer than 44 keys (7 octaves) do not produce sufficient sound and full keyboard range and respond too slowly to be used for serious performances on more than a very limited scale; they are nearly always regarded as toys rather than musical instruments.
Double piano keyboard
Some keyboards, such as the Yamaha P-105 and Kurzweil K2600, have a second set of white keys in the adjacent range that can be independently programmed to specific MIDI notes. Pressing a key on one section of the keyboard tells both sections to play at whichever octave is currently selected.
These extra keys are useful for playing two different parts with two hands at the same time using different voices from the same soundcard. This larger type of piano was incorporated into later home models from Chickering & Sons around 1912 which became standard in uprights by 1918.
The only other musical instrument that normally has double keyboards is the pipe organ. Here, one set of keys plays an upper register (typically played by the left hand) and another set plays a lower range (typically played by the right hand). This arrangement makes it possible to play interlaced parts, one with each hand.
The most advanced digital pianos can have double keyboards so that two people can play on the same instrument at the same time (e.g., one person plays the right-hand part and the other plays the left-hand part). Each player would use a different MIDI channel; this is not an issue since modern digital pianos receive on all channels by default until manually changed.
Simple electronic keyboards often make no distinction between "upper" and "lower" octaves; they may even be missing some keys of each type. On these instruments, some or all notes of the lower octave may be programmed to generate specific timbres for certain patches. Even when the keys are properly divided, some manufacturers program all notes of an octave to sound the same timbre.
Conclusion
Pianos typically have 47 or more keys (seven octaves, from A0 to C7).
However, some pianos may be missing some of the notes in either the upper or lower registers. Home digital pianos often have 61 keys (five full octaves plus a sixth that provides coverage of an additional octave and a half); other home models may only have 73 keys (seven full octaves), while some professional-level instruments may have 88 weighted keys (eight full octaves plus a third pedal keyboard for the low range).
Electronic keyboards designed for musicians frequently feature mini keyboards with just five or six notes and/or contain only one set of speakers to save space and cost.