Two angles are right angles if they are both supplementary and congruent to each other. The Right-Angle Theorem states that if two angles form a straight line and are congruent, then they must each measure 90 degrees and be right angles. The proof shows that two angles <1 and <2 that are supplementary and congruent must each measure 90 degrees by using the definitions of supplementary, congruent, and right angles.