Intervals · Step 9
Perfect, major, minor
Why do some intervals come in two sizes and others in one? Count the seven lettered notes from any starting point and look at the distances. Seconds, thirds, sixths and sevenths arrive in two flavours — a major version and a minor one a semitone smaller — because the letters they span sometimes include a black key and sometimes do not. Fourths, fifths and octaves arrive in one size almost everywhere, and that one size is called perfect.
So a third is major or minor, never perfect; a fifth is perfect, never major. It sounds like bookkeeping, but it is the reason chord names work: the “major” and “minor” in a chord’s name are its third, and only its third. This round mixes the two thirds with the fourth and the fifth — four intervals, two families.