D♯ (D-sharp) or Re Dièse is the fourth semitone of the solfege. It lies a chromatic semitone above D and a diatonic semitone below E, thus being enharmonic to Mi Bémol or E♭ (E flat). However, in some temperaments, it is not the same as E-flat. E-flat is a perfect fifth below B-flat, whereas D sharp is a major third above B.
When calculated in equal temperament with a reference of A above middle C as 440 Hz, the frequency of the D♯ above middle C (or D♯4) is approximately 311.127 Hz. See pitch (music) for a discussion of historical variations in frequency.