Diminished Seventh Chord - Uses

Uses

The most common form of the diminished seventh chord is that rooted on the leading tone; for example, in the key of C, the chord (B, D, F, A♭). So its other constituents are 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. Fétis tuned the chord 10:12:14:17.

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. Because these chords have no leading tone in relation to the chords to which they resolve, they can not properly have "dominant" function. They are therefore referred to commonly as "non-dominant" diminished seventh chords or "common tone" diminished seventh chords. (See "common tone diminished seventh chord" below)

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. The chord, "plays no role in...jazz."

The diminished seventh chord can also be conceived of (and used in practice) as a dominant seventh chord, to which the third, fifth and flat-seventh have been lowered by a semi-tone while the root remains fixed; e.g., lowering the third, fifth and flat-seventh of C7 (C, E, G, B♭) each by a semi-tone yields C, D♯, F♯, A, which is enharmonically equivalent to C, E♭, G♭, B. In this context the diminished seventh chord is instead conceptualized as a dominant thirteen ♯9 ♯11 (e.g., the previous example may be thought of as C13♯9♯11). The diminished seventh chord should not be confused with the half-diminished seventh chord, in which the seventh is not diminished but rather minor (♭7). This said, if any of the four notes in a diminished seventh chord are raised by a semi-tone, that raised note is then the flat-seventh of a half-diminished seventh chord. Similarly, if any of the four notes in the diminished seventh chord are lowered by a semi-tone, that lowered note is then the root of a dominant seventh chord.

The diminished seventh chord comprises frequencies that are equally spaced when considered on a logarithmic axis, and thus divides the octave into four logarithmically equal portions, each being a minor third.

The diminished scale may be conceived of as two interlocking diminished seventh chords, which may be rearranged into the alpha chord.