Sharps and Flats

 
00:00

In music, an accidental is a note whose pitch (or pitch class) is not a member of a scale or mode indicated by the most recently applied key signature. In musical notation, the symbols used to mark such notes, sharps (♯), flats (♭), and naturals (?), may also be called accidentals. An accidental sign raises or lowers the following note from its normal pitch ignoring sharps or flats in the key signature, usually by a semitone, although microtonal music may use "fractional" accidental signs, and 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.

http://en.wikipedia.org/wiki/Sharps_and_flats