Usually 125/128 - the interval between 2/1 and (5/4) * (5/4) * (5/4).
'If you play three major thirds in succesion from C, say, as C E G# B#, using pure ratios for the intervals, the final B# is a quarter tone or so flatter than C - the notes differ by a diesis up to octave equivalence
More generally, a small interval of about a quarter tone 'between low number ratios or products of ratios such as the undecimal diesis 33/32 between 11/8 and 4/3, or 'the septimal diesis 63/64 between 7/4 and 16/9.