An interval between musical notes that spans three whole steps, or six semitones. Two tritones, one on top of the other, span the distance of an octave. The tritone is called an augmented fourth or diminished fifth, depending on its place in a chord or a musical key.
The tritone is one of two strong dissonances in the diatonic scale and many people in the era of early Western music actually called it called diabolus in musica (“the Devil’s interval”). After the advent of equal temperament its characteristic sound was even more pronounced because it was pulled strongly sharp (over 1%) in relation to its natural position in the harmonic series. Composers began to use it extensively because its behavior – which imitated that of a leading tone – was useful to create key changes. It is the only interval in tonal music that keeps its characteristic sound in inversion, such as C to F# or F# to C.