Students learn the difference between a definition and a theorem, as well as the following theorems. If two lines are perpendicular, then they form congruent adjacent angles. If two lines form congruent adjacent angles, then they are perpendicular. If the exterior sides of two acute adjacent angles are perpendicular, then the angles are complementary. Students are then asked to state the definition, postulate, or theorem that justifies given statements, using ideas going back to the beginning of the Geometry course.