Directed Angles and Their Types
Angles can be defined as an ordered pair of rays that meet at a vertex.[more]
The definition is ambiguous as soon as the measure of angle is introduced. Suppose the angle is formed by the positive axis and the positive axis. Is the measure of the angle 90° or 270°?
A small arc going from one ray to the other shows which one is meant.
If the small arc has an arrow, the angle is directed. Switching the arrow gives the negative of the angle and changes the sign of its measure.[less]