diesis

thumb|Diesis on C . thumb|Diesis as three just major thirds.

In classical music from Western culture, a diesis ( or enharmonic diesis, plural dieses ( , or "difference"; Greek: "leak" or "escape" is either an accidental (see sharp), or a very small musical interval, usually defined as the difference between an octave (in the ratio 2:1) and three justly tuned major thirds (tuned in the ratio 5:4), equal to 128:125 or about 41.06 cents. In 12-tone equal temperament (on a piano for example) three major thirds in a row equal an octave, but three justly-tuned major thirds fall quite a bit narrow of an octave, and the diesis describes the amount by which they are short. For instance, an octave (2:1) spans from C to C′, and three justly tuned major thirds (5:4) span from C to B (namely, from C, to E, to G, to B). The difference between C-C′ (2:1) and C-B (125:64) is the diesis (128:125). Notice that this coincides with the interval between B and C′, also called a diminished second.