Geometry · Parallel Lines & Angle Pairs
Co-Interior Angles
HSG-CO.C.9
Definition
Same-side interior angles between two parallel lines that sum to 180 degrees.
Examples
- 120 + 60 = 180 degrees for a pair of co-interior angles on parallel lines.
- Co-interior angles sit between the lines, on the same side of the transversal.
Key rule
Co-interior angles are supplementary when the lines are parallel.
Memory hook
Same side, inside — they add to 180 degrees.