Question

What are alternate interior angles?

Answer 1

Two parallel lines and one transversal intersecting it form `8` different angles.
Two pairs of these angles are called alternate interior.
See picture below.

Explanation:

A pair of angles `a` and `d` are alternate interior.
So is a pair of angles `b` and `c`.

They are called alternate because `a` and `d` (as well as `b` and `c`) lie on opposite sides from transversal `l`.
They are called interior because both `a` and `d` (as well as `b` and `c`) are inside a space between the parallel lines.

Two angles forming an alternate interior pair are always congruent to each other, they have exactly the same measure.