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 cancels a sharp or flat. Accidentals alter the pitch of individual scale tones in a given key signature; the sharps or flats in the key signature itself are not considered accidentals.

An accidental applies to the note that immediately follows it, and to subsequent instances of that note in the same measure unless it is canceled by another accidental. A sharp raises a note's pitch by a semitone and a flat lowers it by a semitone. Double flats () or sharps () may also be used, which alter the unmodified note by two semitones. 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 is not 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.