When a pair of lines is cut by a transversal, eight different angles are created, four around each point of intersection. The angles can be described as interior or exterior depending on where they are located. Interior angles lie inside the parallel lines, while exterior angles lie outside.

Alternate angles are the angles that lie on opposite sides of the transversal. Alternate interior angles are the angles that lie inside the parallel lines on opposite sides of the transversal.

In the figure above, the following angle pairs are alternate interior. $\angle3 \text{ and } \angle 5 \quad \text{and} \quad \angle4 \text{ and } \angle 6$