Exterior Angles of Polygons

The Exterior Angle is the angle between any side of a shape,
and a line extended from the next side.

Note: when you add up the Interior Angle and Exterior Angle you get a straight line, 180°. (See Supplementary Angles)

Polygons

A Polygon is any flat shape with straight sides

The Exterior Angles of a Polygon add up to 360°

 

In other words the exterior angles add up to one full revolution

(Exercise: try this with a square, then with some interesting polygon you invent yourself.)


Note: This rule only works for simple polygons
Here is another way to think about it:
Each lines changes direction until you eventually get back to the start: