Accidentals and Enharmonic Equivalence

The piano keyboard has only twelve pitches in each octave, but music uses seven letter names, A through G. That mismatch is why black keys get two names. The black key between F and G can be called F sharp, because it is a half step above F, or G flat, because it is a half step below G. Same key, same sound, different spelling depending on which letter you are moving from.

Accidentals are the symbols that make these shifts happen. A sharp raises a pitch a half step. A flat lowers it a half step. A natural cancels any sharp or flat and returns a note to its plain letter pitch. A double sharp raises a pitch two half steps, a whole step, and is written as an x. Each accidental attaches to a specific written note and tells you exactly how far it moves from that note's unaltered position.

Spelling choice is not random. Composers pick the spelling that matches where a note came from and where it is going, because the written name signals harmonic role and melodic direction even though the keys and strings cannot tell the difference. This matters once key signatures enter the picture: a key signature sets default accidentals for whole letter names, but any single note can still carry its own accidental that overrides that default for a local reason, which is exactly how a G flat can sit inside a key signature built on F sharp.