When parallel lines are intersected by another line (called a transversal), angle relationships are created. Vertical angles, corresponding angles, alternate interior angles, and alternate exterior angles will be congruent (equal). Adjacent angles, same side interior, and same side exterior will be supplementary which means their sum will be 180 degrees. All of the obtuse angles created by a single transversal will be congruent, and all of the acute angles will be congruent. Two angles whose sum is 90 degrees are called complementary angles.