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.