Diesis (scale theory)

The diesis (precisely the mosdiesis) of a mos scale is the interval that is the difference between two enharmonic notes, i.e. the diminished mossecond or the negative diminished mossecond, whichever is positive. In soft scales, it is the diminished mossecond, since the diminished second itself is negative. In hard scales, it is the negative diminished second. In particular, the diesis or enharmonic diesis in the diatonic scale is the diminished second or the negative diminished second, whichever is positive. For example, the interval of C#-Db or Db-C#.

If the fifth approximates the just interval 3/2, then the diesis or the negative diesis approximates the Pythagorean comma. In meantone, the diesis also approximates 128/125, the augmented comma, which traditionally goes by the name of diesis.