The Tritone, Diabolus in Musica: History of a Feared Interval

Three whole tones between F and B: why medieval theorists avoided the tritone, where the name diabolus in musica comes from and how Romantics used it.

Only one musical interval bears an infernal nickname. The tritone, the distance of three whole tones that separates, for example, F from B, has been called diabolus in musica, the devil in music. Many legends have grown around this name, including the claim that the medieval Church forbade it on pain of punishment. The real story is more nuanced, but no less interesting, and it shows how a technical choice became a symbol.

An Unstable Interval

The tritone divides the octave exactly in half. Unlike the fifth and the fourth, which ancient theorists linked to simple numerical ratios, the tritone corresponds to a complex ratio and sounds unstable, as if asking to resolve into another interval. For this reason, in the medieval musical system, it was perceived as a harshness to be avoided in melodies and in counterpoint.

In the hexachord system, attributed in its essentials to Guido of Arezzo in the eleventh century, notes were sung to the syllables ut, re, mi, fa, sol, la. The tritone appeared when a mi of one hexachord came up against a fa of another. Theorists thus formulated the precept «mi contra fa», meaning that this clash should be avoided. One practical solution was to lower B to B flat, an alteration that belongs to the so-called musica ficta.

The Birth of a Name

Contrary to what is often read, there is no known medieval church decree forbidding the tritone. The interval was discouraged for reasons of theory and taste, not of doctrine. The expression diabolus in musica, too, does not appear in medieval treatises: the known attestations date from the early modern period, when the formula «mi contra fa» was associated with the devil as a schoolroom motto.

The name, then, is more recent than the aversion it describes. But once born, it enjoyed great success. For eighteenth and nineteenth-century musicians the tritone was by now a usable harmonic tension, and its sinister reputation made it an ideal tool for evoking the supernatural, danger and evil.

The Tritone in Romantic Music

In Beethoven's Fidelio, at the start of the second act, timpani tuned a tritone apart, A and E flat, depict the darkness of the prison where Florestan is held. In Weber's Der Freischütz the diminished seventh chord, which contains two tritones, accompanies Samiel's appearances. Liszt opens his Sonata after a Reading of Dante with this very interval, like a door flung open onto hell.

In Saint-Saëns's Danse macabre Death's violin has its E string lowered to E flat, so that the two open strings played together form a tritone. In the twentieth century the interval lost much of its exceptional character, becoming a normal element in the harmonic language of Debussy, Stravinsky and jazz.

The story of the tritone shows how music possesses a symbolism of its own, made of conventions accumulated over centuries. What medieval theorists regarded as a simple defect of consonance became, through theatre and the symphonic poem, the very voice of the devil. It is an example of how musical meanings are not natural, but built by history and shared listening.