Enharmonic equivalents are the same pitch spelled two ways (e.g. C# = Db) — context determines spelling

On a piano keyboard or in equal temperament, C# and Db are produced by the same key and sound identically. They are called enharmonic equivalents. The choice of spelling is context-dependent: in the key of G major (which has F#), that note is spelled F#, not Gb; in the key of F major (which has Bb), the note is Bb, not A#. Each black key has two names — as a sharp of the white key below, or as a flat of the white key above. In equal temperament the pitch difference between enharmonic spellings is zero; in pure tuning a tiny Pythagorean comma separates them.

Examples

D# = Eb, G# = Ab, A# = Bb. In the key of D major, the raised fourth is G# — but the key of Ab major spells it Ab.

Assessment

Name the enharmonic equivalent of each: F#, Bb, C#. Explain when you would use D# versus Eb in a composition.