What are alternate interior angles?

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

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.