From a fixed point O, which we shall call the origin of coordinates, draw three fixed lines Ox, Oy, Oz at right angles to each other as in Fig. 1.1. These will be called the x-axis, y-axis, z-axis respectively and they will be referred to collectively as rectangular cartesian axes Oxyz. The planes Oyz, Ozx and Oxy are called the coordinate planes and they may be referred to as the yz-plane, zx-plane and xy-plane, respectively.