When two parallel lines are cut by a transversal, the two pairs of angles on opposite sides of the transversal and inside the parallel lines, and the angles in each pair are congruent
Two nonadjacent interior angles on opposite sides of a transversal.
A transversal cutting across two or more parallel lines forms equal angles on opposite sides of the transversal and between the parallel lines.
A pair of angles on the inner sides of two lines intersected by a transversal, but on opposite sides of the transversal.
Angles located inside a set of parallel lines and on opposite sides of the transversal.
3 and 4 and 5 are pairs of alternate interior angles in the figure at right (Lesson 4.2).