The perfect fifth is derived from the harmonic series as the interval between the second and third harmonics. In a diatonic scale, the dominant note is a perfect fifth above the tonic note.
The perfect fifth is more consonant, or stable, than any other interval except the unison and the octave. It occurs above the root of all major and minor chords (triads) and their extensions. Up until the late 19th century it was often referred to by its Greek name, diapente, and abbreviated P5. Its inversion is the perfect fourth.