Final answer:
Same-side interior and same-side exterior angles are always supplementary when two parallel lines are intersected by a transversal.
Step-by-step explanation:
When two parallel lines are intersected by a transversal, the pair of angles that are always supplementary are the same-side interior angles and the same-side exterior angles. Supplementary angles are two angles whose measures add up to 180 degrees. In the case of intersecting parallel lines, same-side interior angles like angle 3 and angle 5 or angle 4 and angle 6, as often labeled in diagrams depicting parallel lines with a transversal, are supplementary. Similarly, same-side exterior angles such as angle 1 and angle 7 or angle 2 and angle 8 are also supplementary when the lines are parallel.