a and b are parallel lines. They are cut by transversal c. Drag the blue points on c to move the transversal. [br]How does this affect the angles formed by the intersection of a and b with c? [br]Which angles are always congruent? [br]Which angles are always supplementary?[br][br]Note: Do not allow the blue points to fall inside the parallel lines or the angles will disappear!

This shows that if three parallel lines cut equal segments off the blue transversal then they will also cut equal segments off the red transversal.