Perfect fourth

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Perfect fourth Play (help·info)

The perfect fourth ( Play (help·info)) is a musical interval which spans four scale degrees. It consists of the note and the note five semitones above it on the musical scale. For example, the interval between a C and the next F above it is a perfect fourth; similarly the interval between a G and the next C above it, between an F and the B flat above it, and so on. The perfect fourth may be derived from the harmonic series as the interval between the third and fourth harmonics.

The term perfect identifies this interval as belonging to the group of perfect intervals, so called because they are neither major nor minor (such as thirds, which are either minor or major) but perfect. "Perfect" distinguishes the perfect fourth from the augmented fourth, which is one chromatic semitone larger.

The perfect fourth is occasionally called the diatessaron.^[1] It is abbreviated P4. 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, or about 498 cents ( Play (help·info)), while in equal temperament a perfect fourth is equal to five semitones, or 500 cents.

A helpful way to recognize a perfect fourth is to hum the starting of the "Bridal Chorus" from Wagner's Lohengrin ("Treulich gefuehrt," the colloquially titled Here Comes the Bride), which is a familiar perfect fourth. Another good example is the first two notes of the familiar Christmas carol, Hark! The Herald Angels Sing.

The perfect fourth is a perfect interval like the unison, octave, and perfect fifth, and it is a sensory consonance. In common practice harmony, however, it is considered a stylistic dissonance in certain contexts, namely in two-voice textures and whenever it appears above the bass.^[2] If the bass note also happens to be the chord's root, 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 but one of the strings of a guitar. It is also a very common musical interval to which tom-tom drums are tuned.

	Perfect fourth (equal temperament) The file plays middle C, followed by F (a tone 500 cents sharper than C), followed by both tones together.
Problems listening to this file? See media help.

See also

• augmented fourth
• All fifths
• List of intervals
• list of meantone intervals
• eleventh
• chromatic fourth

References

1. ^ William Smith and Samuel Cheetham (1875). A Dictionary of Christian Antiquities. London: John Murray. http://books.google.com/books?id=1LIPFk6oFVkC&pg=PA550&dq=diatessaron+diapason+diapente+fourth+fifth&lr=&as_brr=3&ei=OVzHSNWIF4-4swP2iYnYDA. 
2. ^ Sean Ferguson and Richard Parncutt (PDF). Composing in the Flesh: Perceptually-Informed Harmonic Syntax. http://smc04.ircam.fr/scm04actes/P43.pdf. Retrieved 2006-09-05.