Same idea as alternate interior angles, but on the *outside*. With a transversal across two parallel lines, the two angles on opposite sides of the transversal outside the parallels are equal.
Drag the ray
118°
obtuse angle
vertical angle = 118° (equal)
Spotting the pair
- Look outside the two parallel lines (above the top one, below the bottom one).
- Take angles on opposite sides of the transversal.
- They're equal — and if they *are* equal, the lines are parallel.
- Each alternate-exterior angle is also vertically opposite an alternate-interior angle, which is why the rule survives.
Your turn
An exterior angle above two parallel lines measures 118°. What's the alternate exterior angle below?
Quick memory aid: alternate ⇒ equal, co-interior (same side) ⇒ add to 180°. Both only hold when the lines are parallel.