How many semitones are there in an octave?

How to get close to this number

Count the keys on a piano within one octave: seven white and five black. That reconstruction is available to anyone who has seen a keyboard, and it is more reliable than recalling a music-theory fact. The repeating visual pattern of the keyboard exists precisely to make this structure legible.

The choice of twelve is not acoustic necessity, and the explanation says so directly. Indian classical music divides the octave into twenty-two microtonal steps, and Arabic maqam systems use quarter tones a piano physically cannot produce. Western tuning is one solution among several to the problem of dividing a frequency doubling into usable steps.

What makes twelve work is that it approximates the simple frequency ratios the ear finds consonant remarkably well for its size — a fifth, a fourth and a third all land close to a whole number of semitones. Equal temperament is a compromise that makes every key slightly out of tune and every key equally usable, which is why it won.