Parallel Lines and a Transversal – Alternate and Corresponding Angles

MathematicsPlane GeometryAges 12–13

Two lines a and b are cut by a transversal line c at points A and B, forming 8 angles. Drag the end of line b to tilt it or make it parallel to a, drag the end of transversal c to change the angle of intersection; the simulation highlights and measures each pair of alternate interior, corresponding, and co-interior angles to show that these pairs are equal (or supplementary) exactly when a ∥ b, and conversely.