Würschmidt comma

The Würschmidt comma is 393216/390625 = |17 1 -8>, an interval of 11.445 cents.

It is the amount by which eight major thirds falls short of a perfect fifth, octave-reduced: ((5/4)^8 * 393216/390625) / 4 = 3/2.

Therefore, it is also the amount by which seven major thirds falls short of 24/5 (i.e., 6/5 plus two octaves). In other words, ((5/4)^7 * 393216/390625) / 4 = 6/5.

Tempering it out leads to Würschmidt temperament. As in meantone, it implies that 3/2 will be tempered flat and/or 5/4 will be tempered sharp, and therefore 6/5 will be tempered flat.