Interval

Interval, the distance between two musical pitches. In Western harmony, the names of intervals reflect the number of notes of the diatonic scale (the seven-note scale using the tones A to G) that are encompassed by the interval. Thus, the interval C-G is termed a fifth, for it encompasses five diatonic-scale notes (intervals are always reckoned inclusively). A unison (Italian unisono, “one sound”) consists of two identical pitches (e.g. two voices singing middle C). An octave (Latin, octavus, “eighth”) consists of two notes that are eight diatonic-scale tones apart (e.g. middle C and the next higher C). Terms such as fifth and third are not precise enough to define all the diatonic intervals fully, and qualifying terms such as major, minor, perfect, and augmented are often added. Unisons, octaves, fourths, and fifths are termed perfect intervals. Their acoustic identity is so strong that if their size is altered perceptibly they lose their essential character. A perfect fourth, for example, C-F, can be analysed to contain two whole tones (C-D, D-E) and a semitone (E-F); the diatonic scale also contains the interval F-B (three whole tones), and the ear perceives it as radically different from a perfect fourth. It is termed an augmented fourth, for it is a semitone larger than a perfect fourth. The other diatonic intervals (seconds, thirds, sixths, and sevenths) appear in two sizes, one of them a semitone larger than the other. The minor third E-G (a semitone, E-F, plus a whole tone, F-G) thus exists alongside the major third C-E (two whole tones, C-D and D-E) to make a C-major chord.

Western music eventually went beyond the diatonic scale in its choice of tonal material, thereby giving rise to other augmented intervals (such as C-A sharp, an augmented sixth, a semitone larger than the major sixth C-A) and to diminished intervals (compressed by a semitone, e.g. C sharp-E flat, a diminished third). Intervals larger than an octave retain the basic acoustic identity of their corresponding smaller intervals; thus, a twelfth is sometimes termed a compound fifth, or an octave plus a fifth.