Tritone

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 .

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.

The tritone interval is used in the musical Deutsch tritone paradox.

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). In classical music Liszt uses the tritone in the same way in "Au bord d´une source" (B flat as dominant for B) and many other places.

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).

The tritone retains its "Devil in Music" character in popular music, specifically heavy metal. The opening of Black Sabbath's signature song Black Sabbath makes heavy use of the tritone. Other metal songs with prominent tritones in their main riffs are Diamond Head's Am I Evil? and Metallica's For Whom the Bell Tolls and Enter Sandman. Other examples are the beginning of Liszt's Dante Sonata and Jimi Hendrix's Purple Haze.