Exterior Angle

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

Triangle vertex showing interior angle and adjacent exterior angle on an extended straight line

Another example:

Polygon side extension showing the exterior angle relationship at a different orientation

Another example:

Polygon side extension showing the exterior angle relationship at a different orientation

When we add up the Interior Angle and Exterior Angle we get a Straight Angle (180°), so they are "Supplementary Angles".

Exterior Angle + Interior Angle = 180°

Example: If the Interior Angle is 70°, what is the Exterior Angle?

We know they add up to 180°:

Exterior Angle = 180° − 70° = 110°