Angles come in pairs with rules: supplementary angles add to 180°, complementary angles add to 90°, vertical angles at a crossing are equal, and adjacent angles share a side and add to the whole. Writing those rules as equations finds an unknown angle. The three angles of a triangle add to 180°, an exterior angle equals the two far angles added, and parallel lines cut by a third make two sets of equal angles. The crew measures the roof frame with a protractor at the Blueprint Table.