In mathematics, the Cartesian coordinate system, or X-Y coordinate system, is used to determine each point in the plane through two numbers, usually called the x-coordinate and the y-coordinate of the point. To define the coordinates, two perpendicular directed lines (the x-axis, or abscissa, and the y-axis, or ordinate), are specified, forming a plane (an xy-plane). Also specified is the unit length, which is marked off on the two axes (see the above figure). Cartesian coordinate systems are also used in space, where three coordinates are used, and in higher dimensions.

Data points to be plotted in the Cartesian coordinate system are written as (x, y), where x is the distance from the y-axis at which the point will be placed, and y is the distance from the x-axis at which the point will be placed. Four examples are given in the figure above.

1. This definition is based on the Wikipedia definition for the Cartesian coordinate system (accessed 27 December 2006) [disclaimer].