The sum of interior angles in a triangle is 180°. If the side of a triangle is extended, an angle is created outside the triangle called an exterior angle.

A square has 4 interior angles, all of equal size 90°.

The sum of interior angles in a polygon can be found by dividing the polygon into a number of triangles.

The exterior angles of a polygon can be found by extending the sides of the polygon to form an angle outside the shape.