What Is an Accidental in Music?

From left to right: Sharp, double sharp, flat, double flat, natural.

Sidney Llyn. About.com, 2017

An accidental in music is a symbol that indicates the modification of a pitch. A music accidental can turn a pitch sharpflat, or back to its natural state. The most commonly used accidentals in music are the sharp (♯), the flat (♭), and the natural (♮). These accidentals raise or lower a pitch by a half-step, making the pitch either higher or lower than it was before the accidental. If an accidental is used on a pitch within a measure, the note with the accidental remains affected by the accidental throughout the measure. To cancel an accidental in the same measure, another accidental, usually the natural sign, must occur within the measure. Black piano keys can also be called accidentals.

How Common Accidentals Work

The sharp accidental (♯) raises a note's pitch by a half-step. A note with a sharp accidental will sound a semitone higher than the same note without a sharp. For example, when notated with a sharp accidental, a C on the piano would become a C♯. Instead of playing the C, you would play the note a half-step higher than C, which is the black key to the right of the C on a modern piano. 

The flat accidental (♭) lowers a note's pitch by a half-step. Any pitch with a flat accidental will cause the note to sound a semitone lower than the same note without the flat. Again using the piano as an example, a B notated with a flat would become B♭. When you see a B with a flat next to the notehead, you would play the note that is a half-step lower than B, resulting in B♭, the black key immediately to the left of the B. 

The natural accidental (♮) can either raise or lower a note's pitch because it cancels previous accidentals to return a note to its natural pitch. In the case of a pitch that has been altered within a measure, the natural sign will cancel the alteration of the pitch. Perhaps there is a measure with a C♯ on the first beat of the measure. If another C is notated in the measure, the C will remain a C♯ unless the natural sign is used on the following C in the same measure to return the C from C♯ to its natural state of C♮. Similarly, the natural sign is often used when a key signature indicates that certain notes are played with recurring accidentals. In the case of F Major, the B will always be played as B♭. However, if a B♮is introduced in the music, it returns the B♭ to its natural state of B♮.

Aside from sharps, flats, and natural signs, there are also double-accidentals in music notation. Although referred to as "accidentals" in English, other musical terms for the accidental is alterazione (It); altération (Fr); and Akzidens (Ger).