PLANE ANGLE

The plane angle XOP, Fig. 1.1, is formed by the two rays OX and OP. The point O is called the vertex and the half lines are called the sides of the angle.

                                                  

More often, a plane angle is thought of as being generated by revolving a ray (in a plane) from the initial  position OX to a terminal position OP. Then O is again the vertex, OX is called the initial side, and OP is called the terminal side of the angle.

An angle generated in this manner is called positive if the direction of rotation (indicated by a curved arrow) is counterclockwise and negative if the direction of rotation is clockwise. The angle is positive in Fig. 1.2(a) and (c) and negative in Fig. 1.2(b). 

Definitions of terms:

INITIAL SIDE 

- a stationary straight line that contains a point about which another straight line is rotated to form an angle

TERMINAL SIDE

- a straight line that has been rotated around a point on another line to form an angle measured in a clockwise or counterclockwise direction

VERTEX

- The point where the rays intersect,