Diminished triad
Two minor thirds stacked on top of each other; the fifth is diminished. Stands in major on the seventh degree.
What it means
The diminished triad consists of two minor thirds stacked on top of each other; between the root and the fifth lies a diminished fifth. In C major it stands as a scale-native chord on the seventh degree: B, D, F. In jazz it rarely appears alone, mostly as part of the diminished or half-diminished seventh chord.
Musical use
Because the diminished fifth is dissonant, the sonority does not rest but demands continuation. In function theory it is interpreted, on the seventh degree in major, as a shortened dominant seventh chord: if one leaves out the root of D7, the third, fifth, and seventh remain — exactly this triad. It contains the tritone of D7 and resolves accordingly into the tonic, the leading tone upward, the diminished fifth together.
On the second degree in minor, by contrast, it belongs to the subdominant and leads to the dominant.
In four-part writing, the diminished triad preferably stands as a six-three chord, that is, with the third in the bass; root position is avoided, because the leading tone in the bass and the diminished fifth above it sound awkward. The third is doubled, not the root, for in major this is the leading tone, whose doubling would lead to parallel octaves.
Extended by a further third, two different seventh chords result: with a minor seventh the half-diminished one, with a diminished seventh the fully diminished one. The difference is audible and decisive for interpretation.
In English it is called the diminished triad.
Other names
Diminuierter Akkord is a designation derived from the English diminished chord. It is less usual in German and can be misleading, because in jazz diminished often means the diminished seventh chord. In chord symbols the triad appears as “dim” or with a small circle.
Performance practice
Interpret the sonority from its context — mostly it represents the dominant. Lead the leading tone upward and the diminished fifth together. And check the position when you write your own settings: as a six-three chord the sonority sounds distinctly better than in root position.