Written music assumes a key signature that raises or lowers certain notes throughout a piece. When a composer needs to alter a single note outside that system, an accidental appears. The sharp raises the pitch by a semitone, the flat lowers it, and the natural cancels either one.

Accidentals apply only to the measure in which they occur, unless tied across a bar line. Performers watch for them carefully because a missed accidental changes the harmony or the color of a melody. In some modern scores, accidentals are written more freely, but the traditional rules still govern most classical and popular notation.

Composers use accidentals to create tension, to modulate briefly, or simply to color a line. Players learn to read them instantly so the flow of the music never breaks.

• Modifies pitch of a single note
• Sharp, flat, or natural symbols
• Usually lasts only one measure
• Essential for chromatic writing