Key idea: Parallel lines and a transversal
When a transversal crosses two **parallel** lines, eight angles form (four at each intersection). They come in special pairs:
- **Corresponding angles**: same position at each intersection (e.g. both top-left). **Equal**.
- **Alternate interior angles**: between the parallel lines, on opposite sides of the transversal. **Equal**.
- **Alternate exterior angles**: outside the parallel lines, on opposite sides of the transversal. **Equal**.
- **Co-interior (same-side interior) angles**: between the parallel lines, on the same side of the transversal. **Supplementary** (sum to 180°).
These are the rules you'll use most. Sketch the figure and label what you know — the rest follows quickly.