Diminished fifth
Six semitones, the fifth reduced by a semitone — sonically the tritone.
What it means
The diminished fifth comprises six semitones; it is the perfect fifth reduced by a semitone. Sonically it corresponds to the augmented fourth, and both together are called the tritone. The older designation “false fifth” means the same interval.
In the notation
It is recognizable by the fact that the two noteheads span five degrees — unlike the augmented fourth, which spans four. The spelling decides the continuation: the diminished fifth draws together, the lower note rises and the upper falls.
Musical use
In the major scale it arises between the seventh and the fourth degree. There it is at the same time the tense interval of the dominant seventh chord (D7), whose resolution carries the cadence: the leading tone rises to the root, the seventh falls to the third of the tonic. The diminished triad on the seventh degree in major contains it between the root and the fifth, as do the diminished and the half-diminished seventh chords.
In modal music it was regarded as something to be avoided between voices; the practice of musica ficta served, among other things, to avoid it by raising or lowering a note.
An entirely different role is played by it in the blues scale, where it joins the minor pentatonic as the so-called blue note and does not call for resolution but remains as a colouring.
The diminished is to be distinguished from the perfect fifth: a single semitone separates the most stable interval after the octave from its tense counterpart.
Other names
Falsche Quint (Latin quinta falsa) is the historical designation of the diminished fifth. Sonically it corresponds, in equal temperament, to the tritone, but is notated and resolved differently: the diminished fifth mostly strives inward, the tritone as an augmented fourth outward.
Performance practice
Lead the notes together — that is the movement from which the effect arises. Take the leading tone close to its goal and the seventh not too high. Derive the interval, when singing, from the dominant seventh chord. And treat the blue note differently: it wants not to be resolved but coloured.