The tritone earned the nickname devil in music, but the church never banned it
Split an octave exactly in half and you land on the tritone, six semitones of restless tension. Theorists labelled it the devil in music, and legend says singers were punished for it. That legend is probably fanciful, yet composers from Liszt to Berlioz happily used the interval to paint Hell.
A tritone spans three whole tones, like F up to B via G and A. In modern equal temperament it measures six semitones, or 600 cents, dividing the 1,200-cent octave precisely down the middle. Strictly speaking only the augmented fourth counts; a broader definition also admits the diminished fifth, so a major scale holds either one tritone per octave or two, depending on whom you ask. In traditional harmony it is a dissonance that pulls hard toward resolution, which is why it sits inside dominant seventh chords and diminished triads.
Medieval church singers mostly avoided it. The first clear prohibition seems to come with Guido of Arezzo's hexachord system, which shifted its note groupings to sidestep the F to B clash. The Latin tag diabolus in musica was originally attached to a different clash altogether: the saying about mi against fa referred to a minor second. Andreas Werckmeister, Johann Joseph Fux, Telemann and Mattheson all repeated the devilish label in the 18th century, usually crediting unnamed older authorities, but no medieval use of the phrase has actually been found.
Baroque and Classical composers tamed the interval, deploying it as the engine of tension and release in tonal harmony. The Romantics then set it loose for its sinister reputation. Liszt used it to evoke Hell in his Dante Sonata; Wagner tuned timpani to C and F sharp for the brooding start of the second act of Siegfried; and Berlioz swung between huge B and F chords as Faust reaches Pandaemonium, a jolt the musicologist Julian Rushton called a tonal wrench.
By the 20th century, twelve-tone and serial composers treated the tritone as neutral. It can even become structural: Ernő Lendvai's axis system, built to explain how Béla Bartók handled tonality, gives the interval a central role.
Source: Tritone
