C♯ (C-sharp) is a musical note lying a chromatic semitone above C and a diatonic semitone below D. C-sharp is thus enharmonic to D♭. It is the second semitone in the French solfège and is known there as do dièse. In some European notations, it is known as Cis. In equal temperament it is also enharmonic with Bdouble sharp (Hisis). When calculated in equal temperament with a reference of A above middle C as 440 Hz, the frequency of C♯4 (the C♯ above middle C) is about 277.18 Hz. See pitch (music) for a discussion of historical variations in frequency.