Angles on Parallel Lines: Which Pair Are You Looking At?
When a transversal crosses parallel lines, several angle relationships appear at once. The safest method is to identify the pair before calculating.
Three common relationships
Corresponding angles are equalAlternate angles are equal
Co-interior angles add to 180°
If one angle is 112°, the co-interior partner is:
180 − 112 = 68°Why diagrams feel confusing
The diagram may show many equal angles. Mark the known angle, then trace the exact relationship you are using.
Exam habit: write the rule name beside the step, such as “alternate angles” or “co-interior angles sum to 180°”.