In modern Western tonal music theory, a
diminished second is the
interval produced by narrowing a
minor second by one
chromatic semitone. It is
enharmonically equivalent to a
perfect unison. Thus, 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.