Scuba divers use a red flag with a white diagonal stripe to let other people know the scuba divers are under water. Which angles must be congruent in order for the top and bottom edges of the flag to be parallel?

The white diagonal stripe creates four angles when it passes through two lines (the top and bottom edges of the flag). To prove that these edges are parallel, an angle's alternate angle (which is above or below its adjacent angle) should be congruent to it.