Diminished seventh chord

The diminished seventh chord is a four-note chord (a seventh chord) composed of a root note, together with a minor third, a diminished fifth, and a diminished seventh above the root: (1, ♭3, ♭5, 7). For example, the diminished seventh chord built on B, commonly written as B^o7, has pitches B-D-F-A♭:

The chord consists of a diminished triad plus the diminished seventh above the root. These four notes form a stack of three intervals which are all minor thirds. Since stacking yet another minor third returns to the root note, the four inversions of a diminished seventh chord are symmetrical. The integer notation is {0, 3, 6, 9}.

Since the diminished seventh interval is enharmonically equivalent to a major sixth, the chord is enharmonically equivalent to (1, ♭3, ♭5, ♮6).

The diminished seventh chord occurs as a leading-tone seventh chord in the harmonic minor scale. It typically has dominant function and contains two diminished fifths, which often resolve inwards.

The chord notation for the diminished seventh chord (assuming root C) is Cdim⁷ or C^o7 (or Cm^6♭5 for the enharmonic variant). The notation Cdim or C^o normally denotes a (three-note) diminished triad, but some jazz charts or other music literature may intend for these to denote the four-note diminished seventh chord instead.

François-Joseph Fétis tuned the chord 10:12:14:17 (17-limit tuning).