What is "diminished seventh"

n. (context music English) A musical interval of the Western twelve-semitone system consisting of nine semitone and spanning seven degrees of the diatonic scale. It is one semitone narrower than a minor seventh and enharmonically equivalent to a major sixth.

In classical music from Western culture, a diminished seventh is an interval produced by narrowing a minor seventh by a chromatic semitone. For instance, the interval from A to G is a minor seventh, ten semitones wide, and both the intervals from A to G, and from A to G are diminished sevenths, spanning nine semitones. Being diminished, it is considered a dissonant interval.

The diminished seventh is enharmonically equivalent to a major sixth. Its inversion is the augmented second.

The diminished seventh is used quite readily in the minor key, where it is present in the harmonic minor scale between the seventh scale step and the sixth scale step in the octave above.

In an equal tempered tuning, a diminished seventh is equal to nine semitones, a ratio of 2:1 (approximately 1.682), or 900 cents. There is no standard just tuning of this interval, but one possibility, assuming the flat submediant is a perfect (5:4) major third below the octave, and the leading tone to be 15:16, would lead to an interval of 128:75, about 925 cents; another interval is 216:125, which is three minor thirds.

The 128:75 just diminished seventh arises in the C harmonic minor scale between B and A by combining B-D, D|F, F-A.