The setup

When a third line (the transversal) crosses two parallel lines, eight angles are created. They group into three named pairs:

• Corresponding angles — same position at each intersection. Equal.
• Alternate-interior angles — opposite sides of the transversal, between the parallels. Equal.
• Co-interior (a.k.a. "same-side interior") — same side of the transversal, between the parallels. Supplementary.

Why it matters

Almost every SAT geometry problem with parallel lines reduces to spotting one of these three pairs.

Worked example

Two parallel lines are cut by a transversal. One angle measures $72°$. Find the co-interior angle on the same side.

$$\text{co-interior} = 180° - 72° = 108°$$

If the diagram doesn't show parallels explicitly, look for matching arrow marks (>) on the lines — that's the convention.