Tritone
A tritone is the interval of six semitones, or three whole tones, which divides the octave exactly in half and is spelled as either an augmented fourth or a diminished fifth.
What does a tritone look like on real notes?
From C, six semitones up is the key between F and G. Spelled F♯, it is an augmented fourth (C–F♯). Spelled G♭, it is a diminished fifth (C–G♭). Same key, same sound, two names.
Inside C major there is exactly one tritone: F and B, the fourth and seventh degrees. F up to B is an augmented fourth; B up to F is a diminished fifth. Those two notes sit together in the dominant seventh chord G7 (G–B–D–F).
How does a tritone sound?
Tense and unresolved, in a way no other interval matches. It sits one semitone above the perfect fourth and one below the perfect fifth, and it sounds like neither.
In G7, the tritone resolves outwards: B rises a semitone to C and F falls a semitone to E, giving the C major chord. That double semitone move is a large part of why the dominant seventh pulls so hard to the tonic.
A well-known melodic example: the song “Maria” from West Side Story opens with a rising tritone on the first two syllables of the name.
Because it is exactly half an octave, the tritone is the only interval that stays the same when inverted. Flip C–F♯ to F♯–C and it is still six semitones.
Was the tritone banned as “the devil in music”?
The Latin nickname diabolus in musica is real. The popular story that the medieval Church banned the interval has no documentary support. Medieval and Renaissance theory treated the tritone as a dissonance to avoid or handle with care in melodies, which is a rule of style rather than a prohibition. Tonal harmony then made it central: every dominant seventh chord contains one.
Try it: Interval Quality plays the major third, perfect fourth, tritone and perfect fifth and asks you to tell them apart by ear.