The angles that lie on the exterior of the intersection created on opposite sides of a paired set of intersecting lines are alternate exterior angles. If the two intersecting lines are parallel, the alternate exterior angles are congruent.
Mathematicians use properties of alternate exterior angles to find the measurement of an angle or to determine the relationships between intersecting lines.
For example, in a drawing of two parallel lines intersected by a transversal, a person uses the alternate exterior angle value to find the measure of an exterior angle. This theorem also solves the measure of a supplementary angle, as well.