tritone

tritone, in music, the interval encompassed by three consecutive whole steps, as for instance the distance from F to B (the whole steps F–G, G–A, and A–B). In semitone notation, the tritone is composed of six semitones; thus it divides the octave symmetrically in equal halves. In musical notation the tritone is written either as an augmented fourth (e.g., F–B or C–F♯) or as a diminished fifth (e.g., B–F or C–G♭).

During the Middle Ages, this interval was considered particularly difficult to sing and was called diabolus in musica (“devil in music”). Until the 18th century, its use in melody was either avoided or carefully limited by rules of counterpoint.