Same Side Interior Angles

Same side interior angles are two angles that are on the same side of the transversal and on the interior of (between) the two lines.

 $If\ l\parallel m,\ then\ m\angle1+m\angle2\ =\ 180\degree$  

Same Side Interior Angles Theorem: If two parallel lines are cut by a transversal, then the same side interior angles are supplementary.

Converse of the Same Side Interior Angles Theorem: If two lines are cut by a transversal and the same side interior angles are supplementary, then the lines are parallel.