accidental

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 the key signature. A note is usually raised or lowered by a semitone, and there are double sharps or flats, which raise or lower the indicated note by two semitones. Accidentals usually apply to all repetitions within the measure in which they appear, unless canceled by another accidental sign, or tied into the following measure. If a note has an accidental and the note is repeated in a different octave within the same measure the accidental is usually repeated, although this convention is far from universal.
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" (more usually sharps), and the white keys are called naturals.

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