A flat is the symbol that lowers a note by one semitone. It looks like a stylized lowercase b and is placed immediately before the note it affects. In a key signature flats indicate which notes are to be lowered throughout the piece unless canceled by a natural.

Flats appear frequently in keys that use the left side of the circle of fifths. Performers must remember the key signature while also watching for additional flats that appear as accidentals. On keyboard instruments a flat usually means playing the black key to the left of the written note, though context can alter that rule.

The flat sign is one of the three basic accidentals, together with the sharp and the natural. Its consistent use across centuries of notation allows musicians to read chromatic music with precision.

Even a single flat can change the color of a chord or the direction of a melody.