Diminished second

In modern Western tonal music theory, a diminished second is the interval produced by narrowing a minor second by one chromatic semitone. In twelve-tone equal temperament, it is enharmonically equivalent to a perfect unison; therefore, it is the interval between notes on two adjacent staff positions, or having adjacent note letters, altered in such a way that they have no pitch difference in twelve-tone equal temperament. An example is the interval from a B to the C♭ immediately above; another is the interval from a B♯ to the C immediately above.

In particular, it may be regarded as the "difference" between a diatonic and chromatic semitone. For instance, the interval from B to C is a diatonic semitone, the interval from B to B♯ is a chromatic semitone, and their difference, the interval from B♯ to C is a diminished second.

Being diminished, it is considered a dissonant interval.



Size in different tuning systems
In tuning systems other than 12-tone equal temperament and its multiples, the diminished second is a distinct interval. It can be viewed as a comma, the minute interval between two enharmonically equivalent notes tuned in a slightly different way. This makes it a highly variable quantity between tuning systems. Hence for example C♯ is narrower (or sometimes wider) than D♭ by a diminished second interval, however large or small that may happen to be (see image below).

In 12-tone equal temperament, the diminished second is identical to the unison, 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. The most commonly used meantone temperaments fall between these extremes, giving it an intermediate size.

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. In general, this applies for all tunings with fifths wider than 700 cents.

The table below summarizes the definitions of the diminished second in the main tuning systems. In the column labeled "Difference between semitones", m2 is the minor second (diatonic semitone), A1 is the augmented unison (chromatic semitone), and S$1$, S$2$, S$3$, S$4$ are semitones as defined in five-limit tuning. Notice that for 5-limit tuning, $1/6$-, $1/5$-, $1/4$-, and $1/3$-comma meantone, the diminished second coincides with the corresponding commas.