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♭ (D flat). It is the second semitone in the French solfege and is known there as Do Dièse.

When calculated in equal temperament with a reference of A above middle C as 440 Hz, the frequency of C♯ is approximately 277.183 Hz. See pitch (music) for a discussion of historical variations in frequency.

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