Two parallel lines are crossed by a slanted line, the transversal, making two crossing points with four angles each. Drag the round handles to change where the transversal crosses each line, which changes its slant. Because the lines are parallel, the angle pattern repeats: at each crossing two opposite angles equal one value and the other two equal its supplement. Equal angles share a color. This shows corresponding angles equal, alternate interior angles equal, and co-interior angles adding to 180 degrees. An info button opens a drawer explaining the relationships.