In geometry, an interior angle (or internal angle) is an angle formed by two sides of a simple polygon that share an endpoint, namely, the angle on the inner side of the polygon. A simple polygon has exactly one internal angle per vertex.
In contrast, an exterior angle (or external angle) is an angle formed by one side of a simple polygon and a line extended from an adjacent side.
The sum of the internal angle and the external angle on the same vertex is 180°.
- Internal angles of a triangle and External angles of a triangle With interactive animation
- Angle definition pages with interactive applets that are also useful in a classroom setting. Math Open Reference