A fifth is the interval between the first and fifth notes of a diatonic scale. A perfect fifth (often abbreviated P5) is a fifth spanning seven half-notes (semitones). For example, the interval from C to G is a perfect fifth, as the note G lies seven semitones above C, and there are five stave positions from C to G. Diminished fifths and augmented fifths span the same number of stave positions, but consist of a different number of semitones (six and eight, respectively).

See also open fifth and parallel fifths.