I was asked an interesting question by the mother of one of my young piano students today.
Why are there the black and white keys on the piano? Not why those colours but why that sort of arrangement.
The best I came up with by way of explanation was that for one thing it helps with navigation and for another if they were all the same side by side you'd need either a very long keyboard or fewer notes to play.

Any better answers both simple and more technical?

I imagine it would be jolly difficult for anyone to stretch an octave if all the notes in between were next to each other (unless the keys were terribly narrow).

Interesting question. I guess it needed to be a repeating recognisable pattern in order to find your way around the keyboard and it made sense to make the 7 white notes the basic major scale of C. The black and white key arrangement seems fairly ergonomically sensible. There were various ultra-complicated designs with split black notes at one time, I believe - (different tunings involved in the days before equal temperament?). I expect Mad Tom will come up with a more eloquent explanation!

I would imagine (though I have no real idea) that the earliest keyboard instruments were diatonic, so only had "white keys" and became more chromatic with the evolution of music, so presumably the first alteration would be a B and a H (or Bb) and so on until we have all 12 notes by about the early 17th century (I think) and that the arrangement has never been changed. Though I fully expect to be shot down by more knowledgable forumites!

This is correct apart from the dates. The earliest keyboards - about 250 BC did indeed have just the white notes of the modern keyboard. Around the 13th century theorists realized that the scale was incomplete, but rather than redesign the keyboard they came up with an ad-hoc solution in which the extra keys were added between existing keys (C-D, D-E, etc.). By 1300 AD the organ keyboard had assumed its modern form, though it was to be another 400 or so years before the system of tuning by even temperament was worked out.

As a design the modern keyboard is awful. A much better design was produced by Paul von Janko in 1882. it allows much wider intervals to be stretched, and allows the same notes to be played in all 12 keys with the same fingering - making transposition trivially easy.

It never caught on because the traditional design was already entrenched and most pianists could not carry their own instrument with them.

Some comments:

Equal temperament was used as a lute tuning by about the late 16th C., and a lute solo exists from c. 1580 that cycles through what we would now call 12 keys before returning to the start.

AFAIK, although it was considered for keyboards during the 18th C., there is no record that anyone used it on clavichords, organs or harpsichords, or even on the early pianos. It became a feasible tuning for keyboard instruments only with the bland tone that piano designers achieved during the first half of the 19th C., by putting felt on the hammer and placing it in the right position on the string to minimise the energy of the fifth partial. This makes the ET major third acceptable. As late as 1870, people with acute ears were complaining about yet another London organ being changed to "that barbarous tuning", equal temperament, in which the instrument sounds out-of-tune in every key (because most organ timbres have strong fifth partials).

Some 17th C. organs and harpsichords had split black keys, giving up to 17 notes to the octave, to allow sharps to be tuned differently from flats.

I had exactly the same question from a young pupil today as I started to introduce sharps....... I gave a similar explanation to you - i.e. to help you find your way round the keyboard more easily - but I guess I'll have to do a little ferreting around in books etc. to try to find when the 'pattern' first came in and why........

oops - have just read the previous 2 posts......

http://www.pianoworld.com/fun/janko.htm

I haven't the foggiest idea how to do links on the forums but at the above address I found pictures of the Janko keyboard.

I take Mad Tom's word for it but it looks awful complicated to me.

Ps Just tried it and it seems to work - the link, not the keyboard I mean. Could you give us a lesson Tom?

At first glance it is complicated, but after a little time staring at it you'll see that it is quite simple and logical. You have three rows each of which is fairly similar to the conventional keyboard. I have never actually played one, so I am trusting second hand opinions, but Artur Rubinstein rated it highly, and everything else I've read says that it is technically superior. There is a good section about it in the famous (and very interesting) book "Men, Women and Pianos"

The trouble is that the technique to play this device will be very different from the keyboard we are all used to and if you switch you lose your existing time investment in the conventional keyboard. Starting from scratch would be different, but where would you find one? This design has suffered the fate common to superior technologies - by the time they are invented an inferior technology is too well established to be supplanted.

Also, the ease of transposition is less of a problem when you can alter the pitch on an electronicv keyboard by twisting a dial, or moving a slider.

However, you can easily demonstrate to yourself how clever it is. Take a simple tune: EDC..., EDC..., GFFE.., GFFE.

Now transpose this to a remote key A#G#F#...,A#G#F#..., C#BBA#, C#BBA#

Same fingering!! You can experiment with all your favourite tunes. Play them in any key. No problem!

Also you can usually find several different patterns for any particular combination of notes, so chords or sequences that are near-impossible on a standard keyboard can become easy.