Dreitone

Dreitone is a term derived from roots meaning "three tones" and is used to denote the interval of three whole tones (six semitones), more commonly known in English as the tritone. It occurs as either an augmented fourth or a diminished fifth (for example, C–F# and C–Gb) and in equal temperament these two spellings are enharmonically equivalent. The interval is symmetrical: its inversion is another tritone.

In tonal harmony the tritone is a highly dissonant and tension-producing interval. It is a defining feature of the dominant seventh chord, where the interval between the chordal third and seventh creates a strong tendency to resolve to a tonic sonority. Because of this tendency it plays a central role in voice-leading and harmonic function in Western classical and popular music.

Historically the tritone carried cautious associations—famously described in medieval sources as diabolus in musica—because of its unstable quality, but composers from the late Romantic era onward used it deliberately for expressive and structural effects. In jazz the tritone underlies the practice of tritone substitution, and in 20th-century and contemporary music it became a common tool for atonality, chromatic harmony, and coloristic effects.

The exact acoustic size of the interval depends on tuning system; in just intonation it can differ from the equal-tempered six-semitone span, affecting its perceived character.