Diminished seventh chord
Three minor thirds stacked on top of each other.
What it means
The diminished seventh chord consists of three minor thirds stacked on top of each other; between the root and the seventh lies a diminished seventh, which sonically corresponds to a major sixth. The completion to the octave is again a minor third, so that the chord divides the octave into four equal parts. As a scale-native chord it stands on the seventh degree of the harmonic minor scale.
Musical use
From the symmetry follows the same property as with the augmented triad: all inversions sound the same, no note stands out as the root, and there are only three different diminished seventh chords. Which note is meant as the root is told only by the spelling.
Functionally the sonority is interpreted as a shortened dominant ninth chord: D7 with a minor ninth, without the root. It represents the dominant and resolves into the tonic, with each of its notes moving on stepwise.
Its actual significance, however, lies in its ambiguity. Since each of the four notes can be interpreted as a leading tone, the same sonority can be resolved, through enharmonic reinterpretation, into four different keys. It is thus the best-known means for rapid modulations into remote keys. In eighteenth- and nineteenth-century music it also appears often at dramatic climaxes, because it produces tension without fixing a goal.
It is to be distinguished from the half-diminished seventh chord, whose seventh is minor and not diminished: this sounds softer and has an unambiguous root.
Since the octave is divided evenly into minor thirds, there are only three sonically different diminished seventh chords. In chord symbols it is designated as dim7 or with a circle and 7. This ambiguity made it, in eighteenth- and nineteenth-century music, a favoured means for surprising modulations and dramatic climaxes, especially in opera and piano music.
Performance practice
Determine the root from the spelling and lead every note on stepwise. With enharmonic reinterpretations, watch the change of spelling in the notated text — that is where the goal changes. And shape the sonority as tension, not as a point of rest.