A point in a two dimensional plane is identified by its x and y coordinates (x,y). In three dimensions, a point is identified by three coordinates (x, y, z). A point can be thought of as a position vector, but in the beginning of this tutorial I will avoid using that name in the interest of separating a point's position from a direction vector. By convention, in three dimensions the x,y plane is the plane of the screen and the z axis is perpendicular to it, with the viewer on the positive z axis.