Perfect fifth

In Westerntonal music, a perfect fifth is the musical interval between two notes separated by seven semitones, usually three whole tones and a semitone; it is the interval from the first to the last of the first five consecutive notes in a diatonic scale. Depending on the tuning system in use, it is likely to be tuned to a frequency ratio of exactly or approximately 3:2.

The perfect fifth (often abbreviated P5) spans seven semitones, (e.g., in C major, C to G); other fifths in diatonic music are the diminished fifth, of six semitones, (e.g., in C major, B to F); and the augmented fifth, of eight semitones, , (e.g., in C melodic minor ascending, E♭ to B).

The perfect fifth may be 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. Until the late 19th century, it was often referred to by one of its Greek names, diapente. Its inversion is the perfect fourth. The octave of the fifth is the twelfth.

A perfect fifth is at the start of "Twinkle, Twinkle, Little Star"; the pitch of the first "twinkle" is the root note and the pitch of the second "twinkle" is a perfect fifth above it.