The tritone (tri- or three and tone) is a musical interval that spans three whole tones. The tritone is the same as an augmented fourth, which in equal temperament is enharmonic to a diminished fifth. However, only the augmented fourth consists of three whole tones in meantone temperament, from where the terminology derives, and calling the diminished fifth a tritone is incorrect in that context. Often, however, writers use the term tritone to mean specifically half of an octave, without regard to what system of tuning it may belong to. Two tritones add up to six whole tones, which in meantone temperament is a diesis less than an octave, but equal to a perfect octave in equal temperament, where the diesis is tempered out. A common symbol for tritone is TT. It is also sometimes called a tritonus, the name used in German. An equal-tempered tritone may be heard tritone.mid.

The tritone is a restless interval, classed as a dissonance in common practice music; less dissonant than the second or seventh, but given the name diabolus in musica ("the Devil's interval") by some from the early music era to the baroque period. It was exploited heavily in the Romantic period as an interval of modulation for its ability to evoke a strong reaction by entering the key least related (retaining only two common tones, the least possible) to what occurs previously.

The equal-tempered tritone (a ratio of $\backslash sqrt\{2\}:1$ or 600 cents) is unique in being its own octave inversion. Note that in other meantone tunings, the augmented fourth and the diminished fifth are distinct intervals because neither is exactly half an octave. In any meantone tuning near to 2/9 comma meantone the augmented fourth will be near to the ratio 7/5 and the diminished fifth to 10/7, which is what these intervals are taken to be in septimal meantone temperament. In 31 equal temperament, for example, the diminished fourth, or tritone proper, is 580.6 cents, whereas a 7/5 is 582.5 cents.

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@KatWithSword:The musical interval of an augmented fourth is sometimes known as tritone and "The Devil In Music" (Diabolus in musica).
scribomancer (Sarah [Pseudonym]) Thu, 10 Sep 2009 12:00:33 -0000
@KatWithSword:The musical interval of an augmented fourth is sometimes known as tritone and "The Devil In Music" (Diabolus in musica).
@kaylakow I "argue" to use augmented fourth.
Davepencilguin (Johhn Godwin) Thu, 10 Sep 2009 03:10:13 -0000
@kaylakow I "argue" to use augmented fourth.

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