# Exterior angle

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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.

If every internal angle of a polygon is less than 180°, the polygon is called convex.

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°.

for example:

## External links

- 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