• Source: Diminished triad

In music theory, a diminished triad is a triad consisting of two minor thirds above the root. It is a minor triad with a lowered (flattened) fifth. When using chord symbols, it may be indicated by the symbols "dim", "o", "m♭5", or "MI(♭5)". However, in most popular-music chord books, the symbol "dim" or "o" represents a diminished seventh chord (a four-tone chord), which in some modern jazz books and music theory books is represented by the "dim7" or "o7" symbols.
For example, the diminished triad built on B, written as Bo, has pitches B-D-F:

The chord can be represented by the integer notation {0, 3, 6}.
In the common practice period, the diminished triad is considered dissonant because of the diminished fifth (or tritone).




Harmonic function



In major scales, a diminished triad occurs only on the seventh scale degree. For instance, 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. This chord has a 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 absent.
On the other hand, in natural minor scales, the diminished triad 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 minor triad found in a major key, the supertonic diminished triad has a predominant function, almost always resolving to a dominant functioning chord.
If the music is in a minor key, diminished triads can also be found on the raised seventh note, ♯viio. This is because the ascending melodic minor scale has a raised sixth and seventh degree. For example, the chord progression ♯viio–i is common.
The leading-tone diminished triad and supertonic diminished triad are usually found in first inversion (viio6 and iio6, 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, the bass resolves up and the upper voices move downwards in contrary motion.




In popular music


Walter Everett writes that "In rock and 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 Daryl Hall's "Everytime You Go Away".
The viio in major keys is relatively less common than the iio, but still does happen. It is almost always used to tonicize the relative minor, in progressions such as viio–V7/vi–vi, which resembles iio–V7–i in the relative minor.




Tuning



In a twelve-tone equal temperament, a diminished triad has three semitones between the third and fifth, three semitones between the root and third, and six semitones between the root and fifth.
In 5-limit just intonation, the diminished chord on VII (in C: B–D–F) is 15:8, 9:8, and 4:3, while on II (in C: D–F–A♭) it is 9:8, 4:3, and 8:5 (135:160:192). According to Georg Andreas 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 1 − D | F, giving a just minor third and Pythagorean minor third (45:54:64) in the notation system used in On the Sensations of Tone as a Physiological Basis for the Theory of Music.
Play , (5-limit major), or on C.




Diminished chord table






See also


Half-diminished seventh chord
Secondary leading-tone chord
Augmented triad
Diminished seventh chord




References



Sources

Roig-Francolí, Miguel (2011). Harmony in Context. New York: McGraw-Hill. ISBN 9780073137940.

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