Definition:Polygon/External Angle

Definition
Contrary to intuition, the external angle of a vertex of a polygon is not the size of the angle between the sides forming that vertex, as measured outside the polygon.

An external angle is in fact an angle formed by one side of a polygon and a line produced from an adjacent side.


 * InternalExternal.png

While $\angle AFE$ is the internal angle of vertex $F$, the external angle of this vertex is $\angle EFG$.

Also see

 * Vertical Angle Theorem, which shows that it does not matter which adjacent side you use, because they are equal.