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Meantone temperaments are musical temperaments;[1] that is, a variety of tuning systems constructed, similarly to Pythagorean tuning, as a sequence of equal fifths, both rising and descending, scaled to remain within the same octave. But rather than using perfect fifths, consisting of frequency ratios of value , these are tempered by a suitable factor that narrows them to ratios that are slightly less than , in order to bring the major or minor thirds closer to the just intonation ratio of or , respectively. A regular temperament is one in which all the fifths are chosen to be of the same size .
Twelve-tone equal temperament (12 TET) is obtained by making all semitones the same size, with each equal to one-twelfth of an octave; i.e. with ratios 12√2 : 1. Relative to Pythagorean tuning, it narrows the perfect fifths by about 2 cents or 1/ 12 th of a Pythagorean comma to give a frequency ratio of . This produces major thirds that are wide by about 13 cents, or 1/ 8 th of a semitone. Twelve-tone equal temperament is almost exactly the same as 1/ 11 syntonic comma meantone tuning (1.955 cents vs. 1.95512).