When a transversal line intersects two parallel lines, such as the case here, exterior and interior angles are formed. Also formed are pairs of angles called corresponding angles. A pair of corresponding angles occupy the same relative position, and are on the same side of the transversal. One is interior, while the other is exterior. From the diagram, the 70° angle and the angle y are corresponding angles, while angles x and z are another pair of corresponding angles. If we know the value for x, we have the value for z. Inspecting angle x, we see that it is adjacent to 70° and together they form a straight line (equal to 180°): 70 + x = 180 x = 180 - 70 = 110 Since angles x and z are corresponding angles, we conclude that: z = 110°