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 $l\parallel m$, then $\angle 1\cong \angle 2$.

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 $l\parallel m$.