Diminished second

In modern Western tonal music theory, a diminished second is the interval produced by narrowing a minor second by one chromatic semitone.

It can be viewed as a comma, the minute interval between two enharmonically equivalent notes tuned in a slightly different way.

[citation needed] In 12-tone equal temperament, the diminished second is identical to the unison (playⓘ), because the chromatic and diatonic semitones have the same size.

In 19-tone equal temperament, which extends 1⁄3-comma meantone, it is identical to the chromatic semitone and is a respectable 63.16 cents wide.

However, in 53-tone equal temperament, which extends Pythagorean tuning, the interval actually shows a descending direction, i.e. a ratio below unison, and thus a negative size, going one step down.