Corresponding Angles Postulate

The Corresponding Angles Postulate states that, when two parallel lines are cut by a transversal, the resulting corresponding angles are congruent.

So, in the figure below, if l || m, then angle1 is congruent to angle2.

Two parallel lines cut by a transversal n, with angles labeled 1 through 8

The converse is also true; that is, if two lines l and m are cut by a transversal in such a way that the corresponding angles formed are congruent, then l || m.