The tritone divides the octave exactly in half (6 semitones) and belongs neither to the consonances nor to the perfect intervals. Its tense, “slippery” sound led it to be frequently avoided in medieval church singing; the nickname diabolus in musica (“the devil in music”) is attested from at least the early 18th century.
In the tonal system the tritone is the engine of harmony: it is contained in the dominant seventh chord (between the chord's third and seventh, i.e. scale degrees 7 and 4), and it is precisely its sharp pull that demands resolution to the tonic. In the 20th century the tritone became an expressive means in its own right — from jazz substitutions to horror soundtracks. And the halving of the octave underlies the whole-tone scale and many symmetrical systems of modern music.
