Tritone

In classical music from Western culture, the tritone ( Play, tri- "three" and tone) is traditionally defined as a musical interval composed of three whole tones. In a chromatic scale, each whole tone can be further divided into two semitones. In this context, a tritone may also be defined as any interval spanning six semitones.

Since a chromatic scale is formed by 12 pitches, it contains 12 distinct tritones, each starting from a different pitch. According to a widely used naming convention, six of them are classified as augmented fourths, and the other six as diminished fifths. In a diatonic scale there is only one tritone, classified as an augmented fourth. For instance, in the C major diatonic scale the only interval formed by three adjacent tones (F-G, G-A, and A-B) is that from F to B.

In the above-mentioned naming convention, a fourth is an interval encompassing four staff positions, while a fifth encompasses five staff positions (see interval number for more details). The augmented fourth (A4) and diminished fifth (d5) are defined as the intervals produced by widening and narrowing by one chromatic semitone the perfect fourth and fifth, respectively. They both span six semitones, and they are the inverse of each other, meaning that their sum is exactly equal to one perfect octave (A4 + d5 = P8). In 12-tone equal temperament, the most commonly used tuning system, the A4 is equivalent to a d5, as both have the size of exactly half an octave. In most other tuning systems, they are not equivalent, and neither is equal to half an octave. The d5 is also called semidiapente.

The tritone is often used as the main interval of dissonance in Western harmony, and is important in the study of musical harmony. "Any tendency for a tonality to emerge may be avoided by introducing a note three whole tones distant from the key note of that tonality."