An exterior angle of a polygon is an angle formed by one side and a line extended from an adjacent side. In this figure, the exterior angles of the polygon are shown in green. Drag the slider to verify that the sum of the exterior angles of a convex polygon is 360°, or press the Play button in the lower left corner to display an animation.

The statement is also valid for non-convex polygons, with the understanding that the exterior angle is negative whenever an angle of the polygon exceeds 180°.