When a line (transversal) crosses two parallel lines, it creates special pairs of angles. These angle relationships — alternate, corresponding and co-interior — are used to find missing angles.
Put your revision into practice
Choose a topic and try a short quiz with instant explanations. Free, with no signup.
Revising angles in parallel lines the night before an exam? These are condensed revision notes — the facts, formulas and mistakes examiners actually mark, with no long explanations. Prefer a full walkthrough with step-by-step reasoning instead? Search “Angles in parallel lines GCSE” on the site for the complete guide.
🔑
Key facts to remember
1Alternate angles are equal (Z-angles): they appear on opposite sides of the transversal.
2Corresponding angles are equal (F-angles): they appear in matching positions at each intersection.
3Co-interior angles (same-side interior or C-angles) sum to 180°.
4Vertically opposite angles are equal wherever two lines cross.
5Angles on a straight line sum to 180°; angles around a point sum to 360°.
✍️
Worked examples
Example 1
Two parallel lines are cut by a transversal. One angle is 65°. Find the alternate angle and the co-interior angle.