In music, an accidental is a note of a pitch (or pitch class) that is not a member of the scale or mode indicated by the most recently applied key signature. In musical notation, the sharp (♯), flat (♭), and natural (♮) symbols, among others, mark such notes—and those symbols are also called accidentals. In the measure (bar) where it appears, an accidental sign raises or lowers the immediately following note (and any repetition of it in the bar) from its normal pitch, overriding sharps or flats (or their absence) in the key signature. A note is usually raised or lowered by a semitone, although microtonal music may use "fractional" accidental signs. One occasionally sees double sharps or flats, which raise or lower the indicated note by a whole tone. Accidentals apply within the measure and octave in which they appear, unless canceled by another accidental sign, or tied into a following measure. If a note has an accidental and the note is repeated in a different octave within the same measure, the accidental does not apply to the same note of the different octave. the first seven letters of the alphabet represent the basic diatonic pitches, with additional symbols called accidentals. In addition to the sharp (♯) and flat (♭) used in Europe to indicate the displacement of a scale degree by a semitone up or down, respectively, Arabic theorists have added accidentals representing a lowering of a pitch by a quarter-tone (flat stroke) and raising it by a quarter-tone sharp (half sharp). (Iranians use different symbols.) The modern accidental signs derive from the two forms of the lower-case letter b used in Gregorian chant manuscripts to signify the two pitches of B, the only note that could be altered. The "round" b became the flat sign, while the "square" b diverged into the sharp and natural signs. Sometimes the black keys on a musical keyboard are called accidentals or sharps, and the white keys are called naturals.

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