Diminished chord

A diminished chord stacks two minor thirds: a root, the note three semitones up, and the note three more semitones above that. The top note is a flat fifth, six semitones above the root, and that squeezed fifth is what makes the chord sound tight and uneasy. Chord charts write it as Bdim or B°.

Where it sits in C major and A minor

Build a chord on each note of the C major scale by stacking every other note (C E G, D F A and so on), and six of the seven come out major or minor. The one on B is B D F: three semitones, then three more. That is vii°, the only diminished triad in C major, and the C major key page shows it in place.

A minor uses the same seven notes, so Bdim shows up there too, this time as ii°. Raise G to G#, as the harmonic minor scale does, and you get a second one: G#dim (G# B D), a vii° that pulls hard into Am.

Bdim rarely sits still. The B wants to rise to C and the F wants to fall to E, so it usually resolves to C. G7 (G B D F) holds the same three notes over a G in the bass, and it is the more common way to get that pull.

Half-diminished and diminished seventh chords

Add a fourth note and the family splits in two:

• Bm7b5 (B D F A) is the half-diminished chord: the diminished triad plus a minor seventh. It is the seventh chord that grows naturally on B in C major, and the usual ii chord in minor-key jazz.
• Bdim7 (B D F Ab) is the fully diminished seventh, three minor thirds stacked evenly. In A minor it appears as G#dim7 (G# B D F), the same four piano keys under different names.

In Gelato Synth, the Dim option on the Chords mode chord-type row turns every pad into a diminished triad on that pad’s root. To see where vii° falls in any key, read How to Find Chords in a Key.