Perfect 4th (P4)

The perfect 4th: 5 semitones, open and sturdy — the interval between adjacent guitar strings.

The perfect 4th spans 5 semitones. In just intonation its frequency ratio is 4:3; in equal temperament it measures 500 cents. Inverted, it becomes the perfect 5th. You can hear it inside chords such as Sus4, 7sus4, 11, m11.