U+1D1D2 Musical Symbol Square B

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In musical notation, an accidental is a symbol that indicates an alteration of a given pitch. The most common accidentals are the flat (♭) and the sharp (♯), which represent alterations of a semitone, and the natural (♮), which indicates no alteration and is often used to explicitly cancel a previous accidental. Accidentals alter the pitch of individual notes, while sharps or flats used in a key signature are not considered accidentals.

An accidental raises or lowers the following note - and any repetition of it in the same measure - from its regular pitch, overriding the key signature. A sharp raises a note's pitch by a semitone and a flat lowers it by a semitone. Double sharps () or flats () may also be used, which alter the unmodified note by two semitones. Accidentals apply to all subsequent instances of the same note within the measure unless canceled by another accidental. If a note with an accidental is tied, the accidental continues to apply, even if the note it is tied to is in the next 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 was not always 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.