The musical interval of a perfect fourth, often P4, is the relationship between the first note (the root or tonic) and the fourth note (subdominant) in a major scale. It is the inversion of the perfect fifth.
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.
A perfect fourth in just intonation most often corresponds to a pitch ratio of 3:4 or 1:1.333..., or various other ratios, while in an equal tempered tuning, a perfect fourth is equal to five semitones, a ratio of 1:25/12 (approximately 1:1.3348), or 500 cents, about 1.955 cents wide.
The perfect fourth is considered the most consonant interval after the unison, octave, and perfect fifth.
Conventionally, the strings of a double bass and a bass guitar are tuned by intervals of perfect fourths, while all strings but one of a guitar are tuned to intervals of perfect fourths.
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