If two parallel lines are cut by a transversal, the consecutive interior angles are supplementary angles.

In the figure above, $\angle 1$ and $\angle 4$ are supplementary as well as $\angle 2$ and $\angle 3.$ $\begin{aligned} m\angle 1 + m\angle 4 = 180 ^\circ \\ m\angle 2 + m\angle 3 = 180 ^\circ \end{aligned}$