Geometry · Parallel Lines & Angle Pairs
Alternate Interior Angles
HSG-CO.C.9
Definition
Congruent angles between two parallel lines on opposite sides of the transversal.
Examples
- 65 = 65 degrees for a pair of alternate interior angles on parallel lines.
- Alternate interior angles form a 'Z' pattern between the two lines.
Key rule
Alternate interior angles are congruent only when the lines are parallel.
Memory hook
Inside the lines, opposite sides — the 'Z' pattern.