A polygon is a flat 2D shape with straight sides, and each corner is called a vertex. The angle inside the shape is an interior angle.

If you extend one side, the angle outside is an exterior angle. At one vertex, the interior and exterior angles form a straight line, so they add to $180^{\circ }$.

You also need three facts from earlier work: angles on a straight line add to $180^{\circ }$, angles around a point add to $360^{\circ }$, and angles in a triangle add to $180^{\circ }$. These facts are the building blocks for polygon questions.