A co-interior angle is formed when two lines are intersected by a third line in two distinct points. The four angles that lie on the inside of the two lines are called interior angles.

The third line that intersects the two lines is called the transversal. Co-interior angles always lie on the same side of the transversal, never on opposite sides. The sum of a pair of co-interior angles is always equal to 180 degrees if the two lines are parallel to each other. When the two lines are not parallel, the sum of the two co-interior angles would not be exactly 180 degrees. This is an important axiom in geometry.