Diabolus in musica
Devil in music, a historical nickname for the tritone.
What it means
Diabolus in musica, Latin for devil in music, is a well-known, popularized nickname for the tritone, the interval of three whole tones, such as F to B. This interval sounds particularly striking and unstable due to its tense, dissonant character.
Written evidence for this nickname is comparatively late, mainly documented from the 18th century onward. A firm, official church prohibition of the tritone, as the popular legend often suggests, cannot be clearly established for the medieval period.
Musical use
Historically, the caution surrounding the tritone stems mainly from compositional reasons: in medieval and Renaissance polyphony, the rule mi contra fa est diabolus in musica held that the note mi (B) should not be set directly against fa (F), because doing so produced a tritone that was difficult to intonate and problematic in voice leading.
In later music history, especially from the Baroque and Romantic periods onward, the tritone was deliberately used as a sonic symbol of unrest, tension, or menace, further reinforcing its popular association with the devil, an association that persists today in film music and popular music.