Co-interior Angles – Same Side, Supplementary

Co-interior angles (also called allied angles) are formed when a transversal crosses two lines: they sit on the same side of the transversal, and between the two lines (interior). Unlike corresponding and alternate angles, co-interior angles are not equal on parallel lines – instead, they are supplementary, summing to exactly 180°.

This is the one relationship in this cluster that follows the “sum to 180” pattern rather than the “equal” pattern, so it's worth double-checking carefully: same side, interior, and supplementary – not equal.

Identifying and Using Co-interior Angles

Real-Life Application

• Ladder against a wall: the ladder and the wall's edge form co-interior angles with the ground as a parallel reference.
• Awnings and canopies: a sloped awning crossing parallel wall lines creates co-interior angle pairs.
• Escalators: an escalator crossing two parallel floor levels forms co-interior angles at each level.

Key Takeaways

Practice: Co-interior Angles