In music notation, a natural is a symbol that nullifies a sharp or flat. It resembles a pair of sevens, the first upside-down and the second right side up, attached to each other front-to-middle. It looks something like this:
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It is sometimes used in a key signature with a piece is written in a minor or diminished key, but usually occurs as an accidental preceding a note.
When a note is modified by a sharp or a flat in a key signature, that change continues throughout the piece; if it this is done as an accidental, it continues for the rest of the measure. In order to cancel this change, the note that was "sharped" or "flatted" is followed by a natural sign, which like other accidentals continues for the rest of the measure.
A natural sign looks something like this, when used as an accidental with a half note (a percent sign is probably the closest ASCII equivalent):
/\
----| /--#-------------------------------------------------
|/
----/------------------------------------------------------
/| # % O
--/-|/\-------------|--------------------------------------
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-|--|--|---#--------|--------------------------------------
\ | | |
---\|_/----------------------------------------------------
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