2/7-comma meantone
In 2/7-comma meantone, the JI fifth of 3/2 is flattened by 2/7 of a syntonic comma (81/80).
It is one of the meantone tunings proposed early on, by Gioseffo Zarlino, Francisco Salinas and Vincenzo Galilei.
It may be described as the regular tuning which tempers out 81/80 and tunes 2/1 and 25/24 purely.
It is also the tuning where the major third (5/4) and the minor third (6/5) are equally off from JI by 1/7 syntonic comma (c. 3.1 cents), with the major third flat, and the minor third sharp.