6 The extremities of a surface are lines. 7 A plane surface is a surface which lies evenly with the straight lines on itself. 8 A plane angle is the inclination to one another of two lines in a plane which meet one another and do not lie in a straight line. 9 And when the lines containing the angle are straight, the angle is called rectilineal . 10 When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right , and the straight line standing on the other is called a perpendicular to that on which it stands.