tritones

A tritone is an interval spanning six semitones, i.e., half an octave. It is the most dissonant interval in the common-practice Western tonal system and tends to demand resolution.

It is enharmonically equivalent to an augmented fourth and a diminished fifth; the two spellings reflect its function in a given context. In equal temperament they inhabit the same pitch class, but the spelling affects how it behaves harmonically.

In tonal music, the tritone appears prominently in dominant seventh chords (for example B and F in a G7 chord), creating tension that resolves to the tonic where the tritone's two notes move by semitone or step to chord tones. The leading-tone and the seventh resolve to the tonic in predictable ways.

Historically, the interval was called diabolus in musica, associated with dissonance and restraint in medieval times; its acceptance grew in the Renaissance and baroque periods as part of functional harmony. In the 20th century and beyond, the tritone is used as a device in jazz, rock, and contemporary classical music, including tritone substitution, where a dominant chord is replaced by the chord a tritone away.

In tuning systems other than equal temperament, the exact size of a tritone can vary; the just-tritone sizes differ from the equal-tempered six semitone span, producing slightly different intervals depending on pitch class.