Intervals: Fourths Through Octaves

Fourths and fifths spanning four and five letter names are usually perfect rather than major/minor: a perfect fourth (P4) is five semitones, a perfect fifth (P5) is seven. "Perfect" here doesn't mean better — it means these intervals stay the same size in both the major and minor scale built on the same root, unlike seconds, thirds, sixths and sevenths, which do change.

Exactly halfway between P4 and P5 sits the tritone (six semitones) — the most unstable interval in Western tonal music, historically nicknamed "the devil in music." It's the engine behind the dominant seventh chord's pull back to the tonic, which you'll meet in the harmony lessons.

Sixths and sevenths follow the same major/minor logic as seconds and thirds: minor sixth (8 semitones), major sixth (9), minor seventh (10), major seventh (11) — each one semitone smaller than a perfect octave (12 semitones), the point where the pattern of twelve pitches closes and repeats.

Go past the octave and you get compound intervals — a ninth is just a second plus an octave, a tenth is a third plus an octave, and so on; they're named and heard the same way, just spread further apart.