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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 1:25/12 (approximately 1:1.3348), or 500 cents, about 1.955 cents wide. A helpful way to recognize a perfect fourth is to hum the starting of the Bridal Chorus (or the colloquially titled Here Comes the Bride), which is a familiar perfect 4th. 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.• 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 strings but one of a guitar.
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