Tritone

A tritone is an interval of six semitones. Its name means three whole tones: C to D, D to E and E to F sharp.

Six semitones also divide the twelve-semitone octave exactly in half. Invert a tritone by moving its lower note up an octave and the result is another tritone. C to F sharp becomes F sharp to C.

That symmetry makes the interval unusual, but it does not tell you where the notes should move. The surrounding chord and the spelling of the notes provide that direction.

It can be either. Count the letter names before looking at the semitone distance.

C to F sharp spans C, D, E, F: four letter names. A perfect fourth would be C to F, so raising F makes the interval an augmented fourth.

C to G flat spans C, D, E, F, G: five letter names. A perfect fifth would be C to G, so lowering G makes the interval a diminished fifth.

On an equal-tempered piano, F sharp and G flat are the same key. On the page, the spelling shows how the note belongs to a scale or chord and often suggests how each voice will continue.

A common diminished fifth is B to F. Move B up one semitone to C and F down one semitone to E. The six-semitone interval contracts to the major third C to E.

Spell and arrange the same pitches as F to B and you have an augmented fourth. Let F fall to E while B rises to C. In that register the interval expands outward to E to C, a minor sixth.

The pitches move to the same destination notes in both examples. The different names describe how the voices are written and heard within their register. This is more useful than memorizing a rule without the notes in front of you.

A G7 chord contains G, B, D and F. B and F form a tritone. They are also the two notes that give the chord its clearest pull toward C major.

B is the leading tone of C major, one semitone below C, so it tends to rise. F is one semitone above E, so it tends to fall. Resolve those two notes and the tritone B-F becomes the stable third C-E.

G can remain as a shared note or move to C, and D can move to C or E depending on the voicing. The essential motion is already present inside B and F. That is why the tritone is often called the engine of the dominant seventh chord.

A tritone does not always have to resolve this way. Music can delay it, redirect it or leave it unresolved. The B-F to C-E motion is the central tonal example because both notes move by the smallest possible step.

The interval appears in several important chord families:

A major or minor triad does not contain a tritone. Add a seventh or alter the fifth, and the interval can become the part that directs the chord toward what follows.

G7 normally leads to C because its B-F tritone resolves to C-E. D flat7 contains F and C flat. C flat sounds like B on a piano, so D flat7 contains the same two sounding pitches, F and B, with their functions reversed.

Because the two dominant chords share that tritone, D flat7 can replace G7 before C. Their roots, G and D flat, are themselves a tritone apart.

The bass motion changes from G to C into D flat to C, a chromatic half step. The shared inner tension still points toward C. This is the practical idea behind tritone substitution, especially in jazz harmony.

When B and F appear inside G7, each note sits one semitone from a note in the C major tonic chord. Your ear has learned the common motion B to C and F to E from tonal music, so holding the tritone delays two expected resolutions at once.

Its tuning also gives it less simple harmonic overlap than an octave or perfect fifth. In equal temperament it sits exactly at 600 cents. Unlike the octave's 2:1 or the pure fifth's 3:2, there is no single simple whole-number ratio that defines every musical use of the tritone.

The old nickname diabolus in musica, or "the devil in music," helped create stories that the interval was banned by the medieval church. There is no solid evidence for a general ban. The useful history is simpler: theorists and singers treated the interval carefully because it was difficult to tune and awkward in certain kinds of counterpoint.