The perfect fifth or diapente is a musical interval which is responsible for the most consonant, or stable, harmony outside of the unison and octave. It is a valuable interval in chord structure, song development, and western tuning systems. The prefix perfect identifies it as belonging to the group of perfect intervals (Perfect fourth, Perfect octave) so called because of their extremely simple pitch relationships resulting in a high degree of consonance.
The perfect fifth is historically relevant because it is the first accepted harmony (besides the octave) of Gregorian chant, a very early formal style of musical composition. The perfect fifth occurs on the root of all major and minor chords (triads) and their extensions. It is one of three musical intervals that span five diatonic scale degrees; the others being the diminished fifth, which is one chromatic semitone smaller, and the augmented fifth, which is one chromatic semitone larger. The Solfege of the perfect fifth is "Do - So". A helpful way to recognize a perfect fifth is to hum the starting of twinkle twinkle little star, which is a familiar perfect 5th. The perfect fifth is abbreviated as P5 and its inversion is the perfect fourth.
In simple terms a perfect fifth can be played on a piano keyboard by holding down two notes, one of which is the seventh note higher than the base note.
 Use in chords
The perfect fifth is a basic element in the construction of major and minor triads, and because these chords occur frequently in much music, the perfect fifth interval occurs just as often. However, due to its high level of consonance, the perfect fifth contributes very little to the overall harmonic effect of any chords containing it (except power chords). Because of this, in any situation that necessitates the omission of notes from a chord (such as for practical reasons of fingering) the note forming the perfect fifth above the chord's root can often be safely omitted, its absence being barely, if at all, noticeable.
A bare fifth, open fifth or empty fifth is a chord containing only a perfect fifth with no third. The closing chord of Mozart's Requiem is an example of a piece ending on an empty fifth, though these "chords" are common in Christian Sacred Harp singing and throughout rock music, especially hard rock, metal, and punk music, where overdriven or distorted guitar can make thirds sound muddy, and fast chord-based passages are made easier to play by combining the four most common guitar hand shapes into one. Rock musicians refer to them as power chords and often include octave doubling (i.e. their bass note is doubled one octave higher, e.g. F3-C4-F4).
 Use in tuning and tonal systems
A perfect fifth in just intonation, a just fifth, corresponds to a pitch ratio of 3:2, while in 12-tone equal temperament, a perfect fifth is equal to seven semitones, or 700 cents, which is equivalent to a pitch ratio of 1:27/12 (approximately 1.4983), about two cents smaller than the just fifth. Because the pitch ratio is a ratio of small numbers, the perfect fifth is harmonically significant.
The circle of fifths is a model of pitch space for the chromatic scale (chromatic circle) which considers nearness not as adjacency but as the number of perfect fifths required to get from one note to another.
 See also
 External link
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