Play two notes on a piano: a C, and the F-sharp just above it.

Listen to that sound. It's tense, unresolved, slightly sinister. It sounds like a question nobody wants answered.

That interval is called the tritone, and it has one of the best reputations in music. For centuries it has been known as diabolus in musica — "the devil in music". You will find it said again and again, in books, documentaries and countless music lessons, that the medieval Church banned it, believing it summoned Satan, and that composers who used it risked punishment.

It's a fantastic story.

There's no evidence it's true.

What the tritone actually is

An octave — from one C to the next — is divided into twelve semitones. The tritone sits exactly halfway: six semitones, or three whole tones (hence the name).

Because it cuts the octave precisely in half, it's an unusual interval. Flip it upside down and you get another tritone. It doesn't belong comfortably to any single key. And it sits in a frequency ratio that the ear hears as unstable — it wants to go somewhere.

That restlessness is the key to everything that follows.

The ban that wasn't

So why did theorists avoid it?

Mostly for practical reasons. Medieval singers learned music using a system of overlapping six-note scales called hexachords, each sung with syllables like ut, re, mi, fa, sol, la — the ancestors of "do, re, mi". A tritone often fell between a mi in one hexachord and a fa in another. It was awkward to find by ear, hard to sing in tune, and didn't fit the system cleanly.

Theorists summed it up in a little Latin rule: mi contra fa. Mi against fa: avoid it.

Where did the devil come from?

The phrase mi contra fa est diabolus in musica — "mi against fa is the devil in music" — is real, but it appears in sources from around the early 1700s, centuries after the supposed medieval ban. By then, it was a colourful teacher's warning, the kind of thing you'd tell students so they'd remember.

Over time, the nickname and the story of a ban merged. The more dramatic version won.

The devil's greatest hits

Once you know the sound, you start hearing it everywhere.

• Black Sabbath, "Black Sabbath" (1970). The riff that arguably launched heavy metal is built on a tritone. It sounds like a door creaking open into something you shouldn't see.
• Jimi Hendrix, "Purple Haze" (1967). The opening clash of notes is a tritone.
• **Camille Saint-Saëns, Danse macabre (1874).** The solo violinist tunes one string down so that Death's fiddle plays a tritone right at the start.
• **Leonard Bernstein, "Maria" from West Side Story (1957).** The first two notes of "Ma-ri-a" form a tritone, which then resolves upward — yearning that finds its answer.
• **Danny Elfman, The Simpsons theme (1989).** The choir's opening "The Simp-sons" leaps up by a tritone.

The same interval can sound evil, ecstatic or comic. It depends entirely on what happens next.

Hearing it differently

Jazz musicians love the tritone so much they built a technique around it. In tritone substitution, a player swaps one dominant chord for another that sits a tritone away — the two chords share the same tense pair of notes, so the substitution works, and it adds a sly, chromatic sound.

Film composers use it for unease and suspense. Metal bands use it for menace. Broadway uses it for longing. And a cartoon uses it to announce a yellow family on a couch.

The verdict

The medieval Church did not ban the tritone. Medieval musicians avoided it because it was difficult, not because it was demonic. The "devil in music" label is real but came later. And the interval itself, far from being exiled, became one of the most important engines of harmony in Western music.

The next time someone tells you about the forbidden chord that scared the Pope, play them the opening of The Simpsons.

Then ask them how scared they are.