[sampo]

> why do pianos have different black and white keys?

The 7 white keys match the C major (same as A minor) scale: C D E F G A B (and C).

I don't know why the ancient musicians thought to make C major as kind of the default scale, but that's how we now tend to see it. You can start a scale from any of the 12 semitones, but piano, and some other instruments, and the traditional musical writing notation are build so that C major is the default, and the remaining 5 semitones (the piano black keys) are written as how they deviate from the default scale.

It's just tradition, I assume.