Diminished & augmented

You know 4+3 (major) and 3+4 (minor). Two combinations remain, and both are symmetric:

DIMINISHED = 3+3: root, ♭3, ♭5. Tense, unstable, spooky — it begs to resolve somewhere. Symbol: ° (C° or Cdim).

AUGMENTED = 4+4: root, 3, ♯5. Dreamlike and floating, common as a passing chord. Symbol: + (C+ or Caug).

Compare with the chords you know: diminished is a minor chord whose 5th dropped a semitone; augmented is a major chord whose 5th rose a semitone.

dim = 3 + 3 · aug = 4 + 4

C diminished: C–E♭–G♭. Each gap is 3 semitones.

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