Perfect fourth

The musical interval of a perfect fourth is the relationship between the first note (the root or tonic) and the fourth note in a major scale.

It can be produced by starting on a high note and playing the fourth below or by starting on a low note and playing the fourth above.

An "ideal" perfect fourth in just intonation corresponds to a pitch ratio of 3:4 or 1:1.333...

In an equal tempered tuning, a perfect fourth is equal to five semitones, a ratio of 1:2^5/12 or approximately 1:1.3348.

Compare with: perfect fifth