A geometrical plane is characterized by the fact that a vector N which is

perpendicular to it at any point is perpendicular to it at every other point. Such a vector N is called a normal to the plane, and apart from the arbitrari-

ness of jNj it is unique apart from its sign (the normal can point to either side of the plane). A plane P is completely determined once a point on the plane

is specified together with a normal vector N.