Diminished Triad

In music, a diminished triad ( Play) is a triad consisting of two minor thirds above the root — if built on C, a diminished triad would have a C, an E♭ and a G♭. It resembles a minor triad with a lowered (flattened) fifth.

In the common practice period, the diminished triad is considered dissonant, or unstable. It lacks tonal center or drive because the diminished fifth symmetrically divides the octave.

A diminished triad occurs in a major scale only on the seventh scale degree; in the key of C, this is a B diminished triad (B, D, F). Since the triad is built on the seventh scale degree, it is also called the leading-tone triad. The leading-tone triad also occurs in the seventh chord built on the fifth degree; in C, this is G dominant seventh (G, B, D, F). For this reason, it has dominant function. Unlike the dominant triad or dominant seventh, the leading-tone triad functions as a prolongational chord rather than a structural chord since the strong root motion by fifth is now absent.

On the other hand, the diminished triad in a minor scale occurs on the second scale degree; in the key of C minor, this is the D diminished triad (D, F, A♭). This triad is consequently called the supertonic diminished triad. Like the supertonic triad found in a major key, the supertonic diminished triad has a predominant function, almost always resolving to a dominant functioning chord.

In a twelve-tone equal tempered tuning, a diminished triad has 3 semitones between the third and fifth, 3 between the root and third, and 6 between the root and fifth. It is represented by the integer notation {0, 3, 6}. In just intonation, the diminished triad on vii is tuned 135:162:160. According to Sorge, the trumpet, in its overtone series on C, gives the diminished triad e-g-b♭ = 5:6:7 ("perfect diminished chord"), but the 7 is too flat and 45:54:64 is preferred. Helmholtz describes the diminished triad as b₁−d | f, giving a just minor third and Pythagorean minor third in the notation system used in On the Sensations of Tone as a Physiological Basis for the Theory of Music.

In some sheet music books, Cdim or C° denotes a diminished seventh chord (a four note chord) with root C, and Cm-5 or Cm♭5 denotes a diminished triad with root C. However, in some modern jazz books and some music theory literature, Cdim or C° denotes a diminished triad, while Cdim7 or C°7 denotes a diminished seventh chord.

"In pop music, the diminished triad nearly always appears on the second scale degree, forming a generally maudlin and dejected iio with its members, 2—4—♭6." Songs that feature iio include Santo & Johnny's Sleep Walk, Jay and the Americans' "Cara Mia", and The Hollies' "The Air That I Breathe". Not so rare but rare enough so as to imply knowledge of and conscious avoidance on the part of rock musicians, examples of its use include Oasis' "Don't Look Back In Anger", David Bowie's "Space Oddity", and two in Paul Young's "Go Away".

The supertonic diminished triad and leading-tone diminished triad are usually found in first inversion (iio6 and viio6 respectively) since the spelling of the chord forms a diminished fifth with the bass. This differs from the fully diminished seventh chord, which commonly occurs in root position. In both cases resolve the bass up and move the upper voices downwards in contrary motion. The popular music symbol for a diminished triad is a capital letter designating the root (as with a major chord) with (♭5) added. For example B(♭5) (see also Chord names and symbols (jazz and pop music)).