C♯ (C-sharp) is a musical note lying a chromatic semitone above C and a diatonic semitone below D; it is the second semitone of the solfège. 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 B (B-double 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 277.183 Hz.^[1] See pitch (music) for a discussion of historical variations in frequency.