In mathematics, the<unk>  (also called rectangular<unk><unk>  is used to determine each point uniquely in a plane through two numbers, usually called the x-coordinate or abscissa and the y-coordinate or ordinate of the point.  To define the<unk> s, two perpendicular directed lines (the x-axis, and the y-axis), are specified, as well as the unit length, which is marked off on the two axes (see Figure 1).<unk> s are also used in space (where three<unk> s are used) and in higher dimensions.
Using the<unk> , geometric shapes (such as curves) can be described by algebraic equations, namely equations satisfied by the<unk> s of the points lying on the shape.  For example, the circle of radius 2 may be described by the equation x2 + y2 = 4 (see Figure 2).
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