Perfect fourth
The
perfect fourth or
diatessaron, abbreviated
P4, is one of two commonly occurring
musical intervals that span four
diatonic scale degrees; the other being the
augmented fourth, which is one
chromatic semitone larger. The prefix
perfect identifies it as belonging to the group of
perfect intervals, so called because of their extremely simple
pitch relationships resulting in a high degree of
consonance. The perfect fourth's
inversion is the
perfect fifth.
Its most common occurrence is between the
fifth and upper
root of all
major and
minor triads and their
extensions.
A perfect fourth in
just intonation corresponds to a pitch ratio of 4:3, while in an
equal tempered tuning, a perfect fourth is equal to five semitones, a ratio of 1:2
5/12 (approximately 1:1.3348), or 500 cents, about 1.955
cents wide.
The perfect fourth is a perfect interval like the
unison,
octave, and
perfect fifth. In
common practice harmony, however, it is actually considered a
dissonance in certain contexts. Specifically, it is considered consonant when heard in its most common position as detailed above; but dissonant when built upon a chord's root. In such a position, the interval's upper note almost always temporarily displaces the
third of any chord, and is then called a
suspended fourth.
Conventionally, the strings of a
double bass and a
bass guitar are tuned by intervals of perfect fourths, as well as all strings but one of a guitar.
*
augmented fourth*
list of intervals*
list of meantone intervals*
eleventh*
chromatic fourth*
Tonalsoft Encyclopaedia of Tuning
