The tritone, which derives its name from the fact that it spans three whole tones, is a musical interval of six semitones. Two tritones add up to an octave. An augmented fourth or diminished or flatted fifth is a tritone. It is abbreviated as π.
One of the two strong dissonances in the diatonic scale, it was called diabolus in musica (The Devil's interval) by some from the early music era to the baroque period. It was exploited more heavily after the advent of equal temperament due to its usefulness to create a modulation. It is the only interval in tonal music that keeps its characteristic sound in inversion.
Under equal temperament, the tritone corresponds to a ratio of √2:1.
The tritone occurs naturally between the 4th and 7th scale degrees of the major scale (for example, in C major F to B), and depending on which of the two notes occurs in the bass, it is either an augmented 4th, or a diminished 5th.
The sound of the tritone is what lends the strong tendency towards resolution that is characteristic of the diminished and Dominant 7th chord.
In jazz harmony, the tritone is both part of the dominant chord and its substitute dominant (also known as the sub V chord). Because they share the same tritone, they are possible substitutes for one another.
For example, in the key of C Major, the primary dominant G7 may be substituted with Db7 which is its substitute dominant. Note that both have the same tritone (B and F, or enharmonically Cb and F in reference to the Db7 chord).
This device can also be used in jazz improvisation, whereupon an improviser may use the chord tones of the Db7 on a G7 chord to create an altered chord characteristic of jazz improvisation.
The Db7 chord tones spell out the b5, b7, b9 and maj3rd of the G7 chord, thus effectively outlining both the guide tones (b3 and b7)of the G7 as well as two altered notes (b5 and b9).
Note: the term "tritone" is also sometimes used in telephony to describe the special information tones, a sequence of three tones played to indicate a technical announcement. This is unrelated to the musical usage.
