Use in Tuning and Tonal Systems
A perfect fifth in just intonation, a just fifth, corresponds to a frequency ratio of 3:2 (702 cents); while in 12-tone equal temperament, where each semitone spans 100 cents, a perfect fifth is equal to 700 cents, about two cents smaller than the just fifth.
The just perfect fifth, together with the octave, forms the basis of Pythagorean tuning. A flattened perfect fifth is likewise the basis for meantone tuning.
The circle of fifths is a model of pitch space for the chromatic scale (chromatic circle), which considers nearness as the number of perfect fifths required to get from one note to another, rather than chromatic adjacency.
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