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In music, the interval of a diminished second is an interval of a minor second, or diatonic semitone, diminished by a chromatic semitone. It is therefore the difference between the diatonic and chromatic semitones, which makes it a highly variable quantity between one meantone tuning and the next. In standard equal temperament, in fact, it is identical to the unison (play (help·info)), because both semitones have the same size. In 19 equal temperament, on the other hand, it is identical to the chromatic semitone and is a respectable 63 cents wide. More typical meantone tunings fall between these extremes, giving it an intermediate size.
Three major thirds in succession plus a diminished second make up an octave, and therefore the diminished second is sometimes considered to be a diesis, which in just intonation is a ratio of 128/125 (play (help·info)). In quarter-comma meantone the diminished second has exactly this value, which is 41 cents, and this may be considered a typical size for it.
The diminished second is significant in relation to musical notation, since enharmonic pairs of intervals, in the sense of intervals which are identical in equal temperament, differ by a diminished second. Hence for example G♯ is less than A♭ by a diminished second interval, however large or small that may happen to be.
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