In classical music from Western culture, a diesis , or "difference"; Greek: δίεσις "leak" or "escape"
is either an accidental , or a very small musical interval, usually defined as the difference between an octave and three justly tuned major thirds , equal to 128:125 or about 41.06 cents. In 12-tone equal temperament 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 spans from C to C′, and three justly tuned major thirds span from C to B♯ . The difference between C-C′ and C-B♯ is the diesis . Notice that this coincides with the interval between B♯ and C', also called a diminished second.