Blue parallel lines cut across by a red transversal
When a transversal cuts across two parallel lines there are 8 angles created, 4 at each intersection. We can consider these in corresponding pairs of angles with one at each junction. There are 4 such pairs: top left, bottom left, bottom right and top right. One pair of corresponding angles, top left (p and t), are active on the animation when you open it. Move points B and C to a variety of positions and note how p and t relate to each other. Use the check boxes to make other pairs of corresponding angles active and see how moving B and C affect these. So what have you deduced about corresponding angles?