Referring to the figure above, the transversal AB crosses the two lines and , creating intersections at E and F. Each pair of alternate exterior angles are outside the parallel lines and on opposite (alternate) sides of the transversal. There are thus two pairs of these angles. Click on the 'next' command to visit both pairs of alternate exterior angles in turn.

Remember: exterior means outside the parallel lines.

If the transversal cuts across parallel lines (the usual case) then alternate exterior angles have the same measure. So in the figure above, as you move points A or B, the two alternate angles shown always have the same measure. Try it and convince yourself this is true. Click on the 'next' command to visit both pairs of alternate exterior angles in turn.

If the transversal cuts across lines that are not parallel, the alternate exterior angles have no particular relationship to each other. All we can say is that each angle is simply the alternate angle to the other.

Drag point P or Q to make the lines non-parallel. As you move A or B, you will see that the alternate exterior angles have no particular relationship to each other.