Diminished 7ths

Two seventh chords grow from the diminished triad:

DIMINISHED 7th (C°7): a perfect stack of minor 3rds — 0, 3, 6, 9 semitones. Every gap is 3. Maximum tension; in old films it plays when the villain appears. Because it is perfectly symmetric, it repeats every 3 semitones.

HALF-DIMINISHED (Cø7, also written Cm7♭5): a diminished triad with a MINOR 7th on top — 1, ♭3, ♭5, ♭7. Softer than the full diminished; jazz uses it constantly in minor keys.

Note: the top note of °7 is labeled 6 here (9 semitones) — in strict theory it is a "double-flat 7", but it is the same key.

°7 = 3+3+3 · ø7 = 3+3+4

C°7: C–E♭–G♭–A. Every gap is exactly 3 semitones.

Practise this in the trainer — it opens with this lesson the first time the concept comes up.