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

More examples:

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

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

When we add up the interior angle and its matching 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's the Exterior Angle?

We know they add up to 180°:

Exterior Angle = 180° − 70° = 110°

Exterior Angles of Polygons Interior Angles Interior Angles of Polygons Supplementary Angles Angles On a Straight Line Straight Angle Degrees (Angle) Geometry Index