idea
Pentatonic and diatonic
If you stop the chain of fifths after five notes, you get the pentatonic scale, the five notes of the black keys. It appears in songs from China, Scotland, West Africa and the Andes. If you stop after seven notes, you get the diatonic scale, the white keys. Both scales have something in common: their steps come in only two sizes, so the notes are spread fairly evenly around the octave. This may be one reason why they are easy to sing and remember, and why they appear in so many places.