The interval is the distance between two consecutive tones (source and target tones).

Only the pure as well as the big and small intervals are diatonic (build with tones in the scale only); the diminished and augmented intervals are chromatic. Chromatic intervals regularly lead to enharmonic change.

The complementary interval is the interval between the target tone of an interval and the next octave above the source tone. (The sum of the level of interval and inversion interval always gives 9: second = 2 + seventh = 7 = 9 etc.) Pure intervals have pure complement intervals, major ones have minor ones and vice versa, diminished ones have augmented ones and vice versa.

Intervals and complementary intervals in C major
Above: interval; below: complementary interval
* = major; ** = minor
- = diminished; + = augmented. Enharmonic change: E# = F, B# = C

Any interval exceeding one octave is considered a compound interval. Its complementary interval corresponds to the complementary interval of the interval added to the octave.