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I know that the piano contains 7 octaves plus a 4th. I also know that it accommodates the full circle of fifths (from C and back to C). But since a fifth contains 7 semitones, it only "needs" 7X12 = 84 keys for the circle. So why the "extra" 4 keys?

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up vote 9 down vote accepted

Well, the piano started out with only about 60 keys, same as the harpsichord – in fact it WAS a harpsichord, except that the harpsichord maker Bartolomeo Cristofori (try saying that 10 times fast!) got the bright idea of putting hammers on one (to HIT the strings) instead of plectra (to PLUCK the strings).

So the piano was invented – this was around 1700, or maybe a little before that. Anyway, as composers began to use the new instrument they started writing more and more complicated and brilliant music for it. Pretty soon, the keyboard had to expand in both directions. By the middle of the 19th century, it had 85 notes – up to A – then finally they added the last three at the top. There's even a piano made today – the Bösendorfer Imperial Concert Grand – which has 97 keys; the bass notes go all the way down to C. It's nine-and-a-half feet long and weighs almost a ton.

Piano Education - FAQ

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@Tom: The Bösendorfer Imperial has 97 keys. It has been praised by many artists of the 19th century. The added strings, weight of table, and resonance may have pleased them. There is a cache and these 9 more keys are of a different color so that they do not disturb the player too much. –  ogerard May 9 '11 at 13:51
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Is that site a reputable source? –  billynomates May 9 '11 at 15:26
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@billynomates: Written by John M. Zeigler, Ph.D. and William Leland, D.M.A., R.P.T.; Rio Rancho, NM USA. –  Tom Wijsman May 9 '11 at 15:58
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This answer is interesting, but doesn't seem to answer the actual question of why they added the extra four keys –  BlueRaja - Danny Pflughoeft May 9 '11 at 16:48
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This site seems to be correct. Certainly, when Bartolomeo Cristofori invented the piano (fortepiano specifically) in ~1699, it had the standard 60 keys of a harpsichord. –  Noldorin May 10 '11 at 16:29
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There are a lot of explanations for this, such as "The extra 4 keys allow the piano to cover the entire range of a 'standard' orchestra." However, it's probably just tradition. The human ear can't really distinguish notes higher or lower very well, but it provides more functionality than 4- or 5-octave pianos.

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The keyboard grew beyond five octaves as piano makers essentially said "Ours is better because it has more notes."

There is no particular reason why it didn't stop at 85 or keep going to 90. It could have done either. Eventually the market required that all pianos have the same number, and it just happened to stabilize at 88.

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Do you have some article links to back up these statements? –  awe May 20 '11 at 9:50
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