Diesis (scale theory)

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The diesis (precisely the mosdiesis) of a mos scale is the interval that is the difference between two enharmonic notes, i.e. the diminished mosstep or the negative diminished mosstep, whichever is positive. In soft scales, it is the diminished mosstep. In hard scales, it is the negative diminished mosstep, since the diminished mosstep itself is negative. 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.

See also