When two parallel lines are cut by a transversal, due to the natures of the different relationships among the angles, all of them will be either equal to the measure of the first angle or equal to that angle subtracted from 180. If the transversal is perpendicular to the parallel lines, forms a right angle, then both the angle given and the angle subtracted from 180 will be 90. This means all of the angles will be 90.