Alternate Interior Angles
The term alternate interior angles is often used when two lines are cut by a third line, a transversal.
In the figure above, line t is a transversal cutting lines k and l, and there are two pairs of alternate interior angles:
2 and 8
3 and 5
The Alternate Interior Angles Theorem states that if k and l are parallel, then the pairs of alternate interior angles are congruent. That is,
2 
8
and
3 5.
The converse of this theorem is also true.
