A perfect fifth is a fifth that spans seven semitones. For example, the interval from C to G is a perfect fifth because G lies seven semitones above C. This contrasts with the diminished fifth, which spans six semitones, and the augmented fifth, which spans eight semitones.
The pitches of a perfect fifth have a frequency ratio of exactly or nearly 3:2. It is considered more consonant, or stable, than any other interval except the unison and the octave. Perfect intervals, which also include fourths and octaves, are described as perfect both because of their high degree of consonance and because their inversions are also perfect.