81/70, the septimal semiaugmented second or septimal ultramajor second is a 7-limit interseptimal ratio of about 253 cents. It is sharp of a major second 9/8 by a septimal quartertone 36/35, sharp of a supermajor second 8/7 by a syntonic comma 81/80, and flat of a subminor third 7/6 by a sensamagic comma 245/243.
It is also flat of a minor third 6/5 by a subminor second 28/27. For this fact it is useful in the Canovian chord and provides the function of a voice leading up to the minor third. The canou temperament targets this progression and uses it as one of the generators.
The interval is so perfectly approximated by 19edo, with an error of 0.05 cents. There are a number of edos that do this equally well, 171edo to name one. The first edo that does this better than 19edo with patent val is 660edo.