Notes, octaves and the alphabet of music

Western music uses 12 distinct pitches that cycle endlessly: C, C#, D, D#, E, F, F#, G, G#, A, A#, B. After B, we're back at C - one octave higher.

An octave is the interval between a note and the next note with the same letter name. Acoustically, the higher note vibrates exactly twice as fast as the lower one. Our ears hear them as 'the same note'.

Why 12 notes?

It's a cultural choice that worked beautifully. Other traditions divide the octave differently - Indian classical uses 22 microtonal shrutis; Arabic maqam uses quarter tones.