Sometimes it is necessary to use notes which are not within the key signature of a piece of music. An accidental is a temporary alteration to the pitch of a note. An accidental may be a sharp, a flat, or a natural.
A natural sign is used to cancel a sharp or flat. Like a sharp or flat, the natural affects all notes of that pitch for the rest of the bar in which it occurs.
This example demonstrates accidentals applied to the note G in the treble staff.