In music theory, a diminished seventh is an interval encompassing nine semitones, or a particular chord containing this interval. The interval of a diminished seventh is an interval spanning seven scale degrees and containing nine half steps. It is one half-tone smaller than a minor seventh and 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. A diminished seventh chord is a seventh chord comprising a diminished triad plus the interval of a diminished seventh above the root. The most common form of the diminished seventh chord is one which includes the leading tone, as well as the second, fourth, and flatted sixth (flat submediant) scale degrees. These notes occur naturally in the harmonic minor scale, but this chord also appears in major keys, especially after the time of Bach, where it is "borrowed" from the parallel minor. Seventh chords may also be rooted on other scale degrees, either as secondary function chords temporarily borrowed from other keys, or as appoggiatura chords: a chord rooted on the raised second scale degree (D♯-F♯-A-C in the key of C) acts as an appoggiatura to the tonic (C major) chord, and one rooted on the raised sixth scale degree (A♯-C♯-E-G in C major) acts as an appoggiatura to the dominant (G major) chord. These chords may be referred to as "secondary diminished seventh chords" or as a subclass of secondary dominants. In jazz, the diminished seventh chord is often based on the lowered third scale degree (the flat mediant) and acts as a passing chord between the mediant triad (or first-inversion tonic triad) and the supertonic triad: in C major, this would be the chord progression E minor - E♭ diminished - D minor. |
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