C Sharp as an Enharmonic Note

C sharp and D flat name the same pitch under the twelve-tone equal temperament tuning used in virtually all modern Western instruments, a relationship called enharmonic equivalence: the piano key that sounds this pitch is a single key regardless of which name a piece of written music gives it. Which name a composer or performer uses is generally decided by the key signature and the harmonic context of the passage rather than by any property of the pitch itself, since a note functioning as the raised third degree of a chord is more naturally read and written as one name and the same pitch functioning as the lowered second degree of a different chord is more naturally read as the other.

Before equal temperament became standard, older tuning systems such as meantone temperament did not treat C sharp and D flat as identical in pitch at all, tuning them very slightly apart; the now-familiar idea that every enharmonic pair sounds exactly the same is itself a consequence of the specific tuning compromise equal temperament represents, adopted across most of Western music only over the seventeenth through nineteenth centuries.