A concert piano has eighty-eight keys: fifty-two white and thirty-six black, from the lowest A to the highest C, seven and a quarter octaves. It is a number that seems natural and is not. It is the result of a century and a half of commercial warfare between factories, and of two physical limits beyond which building no longer paid.
At the beginning there were far fewer keys. The instrument Bartolomeo Cristofori perfected in Florence around 1700, the gravicembalo col piano e forte, has little more than four octaves. The pianos Mozart played had five, from F to F, and this explains a great many things in his sonatas: certain phrases that seem to fold back suddenly are not expressive choices, they are notes that were not on the instrument.
Then the race begins. Beethoven fuels it in person, because he constantly writes beyond the available limit and the builders chase him. In 1818 Broadwood, from London, sends him as a gift an instrument with six octaves, and he uses it to the full. Every extra octave was a selling point: the factory that offered more range sold more, and in the nineteenth century the firms outdo one another up to six and a half octaves, then seven.
The race stops towards the end of the nineteenth century, and the number it stops at is eighty-eight. From then on Steinway and the other great factories standardise, and in a century and a half that measure has not moved. There are two reasons why it has not moved, and neither is musical in the strict sense.
The first concerns the frame. Every string under tension pulls, and the sum of those pulls on a concert piano comes to several tens of tons, borne by a cast-iron plate. Lengthening the keyboard means adding strings, hence tension, hence weight and size, hence a case that will not fit in homes and is costly to move in theatres. There is a point beyond which the increase does not pay for itself.
The second concerns the ear, and it is the more interesting one. Below a certain frequency the perception of pitch degrades: you keep hearing the sound, but it becomes hard to tell which note it is, because the fundamental is weak and the ear works on the harmonics. In the last low notes of a piano this is already perceptible, and they are notes used for weight, not for melody. At the opposite extreme the mirror image occurs: in the highest octave the strings are very short, the sound dies almost at once and precise intonation becomes a problem. Those notes serve as flashes, not as song. Adding more, above or below, would have produced keys that nobody would use.
Someone has tried anyway. The Viennese house of Bösendorfer has built for over a century the Imperial model, which goes lower and reaches ninety-seven keys. The extra keys are often coloured black or covered, so that the pianist does not get lost looking for the wrong C. They serve mainly for an effect of resonance: you hold them down in silence and the long strings vibrate sympathetically beneath the rest of the piece. A couple of contemporary builders have gone beyond eight octaves, but they remain collector’s curiosities.
The practical consequence concerns those who play early music. A Haydn sonata performed on a modern piano comes out with an abundance of resources the composer did not have, and with a weight in the bass that the writing did not foresee. It is not a mistake, it is a translation, and you should know that you are translating.
So eighty-eight is not a perfect number. It is the point at which, around 1880, the cost of adding strings overtook the advantage of having them, and since then nobody has had a strong enough reason to reconsider the piece of furniture.