Consider a pair of lines cut by a transversal. The pairs of interior angles with different vertices that lie on the same side of the transversal are called same-side interior angles or consecutive interior angles. Alternatively, same-side interior angles are called co-interior angles. In the diagram, two pairs of same-side interior angles can be identified. Pair1: & ∠ 3 and ∠ 6 Pair2: & ∠ 4 and ∠ 5 If two parallel lines are cut by a transversal, then the consecutive interior angles are supplementary. The same logic in reverse can be applied. If two lines and a transversal form consecutive interior angles that are supplementary, then the lines are parallel.

If	Then
l_1 ∥ l_2	m ∠ 3+m ∠ 6=180^(∘) and m ∠ 4 + m ∠ 5 = 180^(∘)
m ∠ 3+m ∠ 6=180^(∘) or m ∠ 4 + m ∠ 5 = 180^(∘)	l_1 ∥ l_2

These statements are supported by the Consecutive Interior Angles Theorem and its converse.