Axes used in rectangular coordinate systems. Those in the top illustration are used in plane geometry, the x-axis being at right angles to the y-axis. Those in the bottom illustration are for use in three dimensions, the x-, y, and z-axes being at mutual right angles.
Rectangular coordinates are a set of coordinates that gives the position of an object with respect to two or three mutually perpendicular axes (x, y, and z) that cross at some specified origin. In astronomy, heliocentric or geocentric rectangular coordinates are occasionally used to give positions of bodies in the Solar System.