The Number line reference article from the English Wikipedia on 24-Jul-2004
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Number line

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A number line is a one-dimensional graph. It is often used as an aid in teaching simple addition or subtraction, especially involving negative numbers.

The number line is most often represented as being horizontal (though it need not be so). It is divided into two symmetric halves by the origin, i.e. the number zero. Customarily, positive numbers lie on the right side of zero, and negative numbers lie on the left side of zero (though it need not be so).

See also: real number, extended real number line.