A transversal cutting two parallel lines makes a famous Z-shape. The two angles tucked into the inside of the Z — on opposite sides of the transversal, between the parallels — are equal.
Drag the ray
65°
acute angle
vertical angle = 65° (equal)
How to find the pair
- Look between the two parallel lines (the 'interior').
- Pick angles on opposite sides of the transversal.
- Trace the shape — it makes a Z (or backwards Z). Those two angles are equal.
- It works in reverse: if a pair of alternate interior angles is equal, the lines must be parallel.
Your turn
A transversal crosses two parallel lines. One interior angle is 65°. What is its alternate interior angle?
Watch out
This only works when the lines are parallel. If the two lines aren't parallel, alternate interior angles are *not* equal — there's no Z-rule.