\begin{definition}[Definition:Polygon/External Angle]
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.
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While $\angle AFE$ is the internal angle of vertex $F$, the '''external angle''' of this vertex is $\angle EFG$.
\end{definition}