A tritone is an interval comprised of three whole tones (six semitones) resulting in an augmented fourth or diminished fifth. In the major scale it occurs between the fourth and seventh notes (subdominant and leading notes), and so too in the minor scale, except in the descending form of the melodic minor. The tritone is the only interval that is the same as its inverse. It is a difficult interval to sing and hence usually avoided in composing vocal music.
The Devil's interval
Medieval musicians branded the tritone Diabolus in Musica or the Devil in Music, and it was banned from use in religious songs. However, the Devil's interval enjoyed great popularity among composers in the 19th century, who used it to evoke a spooky or malicious atmosphere. In Wagner's Gotterdammerung it is used in one of the exciting scenes – a pagan, evil scene. The tritone has also been used by heavy metal band Black Sabbath, most notably in their eponymous song, as well as in West Side Story's Maria, The Simpsons theme tune, and Jimi Hendrix's Purple Haze.