The perfect fourth, also known as P4, is an interval on the fourth natural. For instance C to F or G to C. The perfect 4th can be augmented: C to F#. Or diminished: C to Fb. But a fourth can never be minor or major. It is perfect because it has 5 semitones between the notes.

This also explains why (for example) F to B is not a P4. There are 6 semitones between F and B. Thus the perfect 4th from F would be a B flat.

Please note that while an augmented third (C to E#) may sound the same as a perfect fourth, it is theoretically not the same and will never be written and referred to as such. This is because an augmented third is on the third natural (C-D-E) and, although both intervals have 6 semitones, can therefor never be called a fourth. The same would apply for F to A#.

History

The name perfect origins from the medieval era. Where the prime, fourth, fifth and octave were indicated as the most consonant intervals.

Sound

The P4 on its own or in a melody sounds quite stable and neutral. But, when using it in a chord like sus4 can become a bit more dissonant and unstable. This is mainly because the perfect fourth is only one semitone above the major third, which asks for a tension release downwards. This tension is less strong on a minor chord.

The sound becomes very unique when stacking only perfect fourths to create a fourth chord. A chord that sounds very neutral, stable, but also a bit twangy. And emphasises the multitude of characters this interval can have.

Usage

The perfect fourth can be used in melodies and chords, both in major a minor keys. It is however not a chord-note, like the tonic, third and fifth. But can be added or suspended into a chord.

Leaving out a major third and replacing it for a P4 asks strongly for a release downwards. For example, listen to the guitar intro from Queens: Crazy little thing called love. Which uses the progression D Dsus4 D.

It is never used at the same time with a major third in a chord because it creates too much dissonance. However, the m11 chord shows that on a minor chord, things are a bit different. Next to that: a sus4, m11, 11 chord is never used as an ending in a song or composition because it will not sound finished.

Using the interval as an added note makes it a suspended fourth (or sus4) when playing it under the octave. Above the octave it will be referred to as an 11th. For example m11 (a minor chord with a m7 and 11).

Checkout our Spotify Playlist with examples how to recognize a perfect fourth!