625/624, the tunbarsma, is an unnoticeable 13-limit comma with a value of roughly 2.77 cents. Tempering out this comma not only equates 25/24 with 26/25 – splitting 13/12 in half and going part of the way towards splitting the 9/8 whole tone into three (the other comma that needs to be tempered in order to finish this job is 676/675) – but also equates 39/32 with the interval that results from stacking four 5/4 major thirds then octave-reducing.
In terms of commas, it is the difference between
It can be factored into
In the 17-limit, it can be factored into