Key Concepts: Parallel Lines and Transversals

Parallel Lines

Parallel lines are coplanar lines that never intersect. Symbol: ∥ (e.g., Line l ∥ Line m).

Transversal

A transversal is a Line that intersects two or more coplanar lines at different points. When a transversal crosses two lines, it creates 8 angles.

Angle Pairs Formed by a Transversal

• Corresponding angles: Same position at each intersection (e.g., both upper-left). ∠1 and ∠5.
• Alternate interior angles: On opposite sides of the transversal, between the lines. ∠3 and ∠6.
• Alternate exterior angles: On opposite sides of the transversal, outside the lines. ∠1 and ∠8.
• Consecutive interior angles (co-interior / same-side interior): Same side of transversal, between the lines. ∠3 and ∠5.