Do you see any similarity among Black Sabbath, Wagner's Gotterdammerung, West Side Story and the theme song of The Simpsons?
All rely on tritones. This restless, dissonant interval spans three whole tones (for example the diminished fifth or augmented fourth). Dating from the Middle Ages, the tritone was the development of Guido of Arezzo, nicknamed Diabolus in Musica, meaning the Devil in music
A rich myth surrounds the tritone—that the Church attempted to eradicate the sounds from its music because it invoked a sexual response and was genuinely the work of the Devil.
In an article on BBC News, Professor John Deathridge, King Edward professor of music at King's College London, states:
"In medieval theology you have to have some way of presenting the devil. Or if someone in the Roman Catholic Church wanted to portray the crucifixion, it is sometimes used there."
"But there were musical treatises and sets of rules produced that did come to forbid the use of the interval, which was seen as wrong when it came up in choruses of monks.
"There are strict musical rules. You aren't allowed to use this particular dissonance. It simply won't work technically, you are taught not to write that interval. But you can read into that a theological ban in the guise of a technical ban."
In the Romantic period, composers began using the tritone freely, exploiting the evil connotations. Liszt uses it to suggest hell in his Danta Sonata. Tritones were important in the development of jazz tertian harmony.
But to this day, the much maligned tritone suggests an evil, oppressive or scary meaning in music.