Uncheck the box for forcing parallel lines and check the box for vertical angles. Drag one of the points on the parallel lines to change the slope of the two parallel lines. Describe the pairs of vertical angles for both sets of lines.
For any two intersecting lines, there will be two pairs of vertical angles which are congruent. When a transversal is intersecting two or more lines that are not parallel, each intersection will contain two sets of congruent vertical angles but the other line will contain different sets of congruent vertical angles.