Given two parallel lines and a transversal, which of the following pair of angles are equal?
Alternate interior angles
Alternate exterior angles
The pairs of angles on the opposite sides of the transversal but inside the two lines are called alternate interior angles. When there are two parallel lines and one transversal line then these alternate interior angles are equal.
Since alternate interior angles are equal, so are the alternate exterior angles (Linear Pair Axiom). See the figure given below.