In classical music from Western culture, a fourth is a musical interval encompassing four staff positions (see here for more details), and the perfect fourth ( Play (help·info)) is a fourth spanning five semitones. For example, the ascending interval from C to the next F is a perfect fourth, as the note F lies five semitones above C, and there are four staff positions from C to F. Diminished and augmented fourths span the same number of staff positions, but consist of a different number of semitones (four and six).
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.
Up until the late 19th century, the perfect fourth was often called by its Greek name, diatessaron. 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 geführt", the colloquially-titled "Here Comes the Bride"). Other examples are the first two notes of the Christmas carol "Hark! The Herald Angels Sing", and, for a descending perfect fourth, the second and third notes of "O Come All Ye Faithful".
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, in the terminology used in popular music, is then called a suspended fourth.
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