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In 1/3-comma meantone temperament, each perfect fifth is tempered by a third of a syntonic comma from its just value of 3/2. This results in minor thirds being exactly 6/5 (and major sixths being exactly 5/3).
In other words, 1/3-comma meantone is the specific tuning of meantone temperament with an eigenmonzo of 6/5.

The fifth is 694.786 cents in size, and the major third 379.145 cents, which is about 7 cents flatter than in 1/4-comma meantone.

1/3-comma meantone is practically identical to the closed system of 19edo (although technically there is a slight difference).

External links

Die mitteltönige Stimmung mit reinen kleinen Terzen (explanation of 1/3 comma meantone, in german)