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 equal
Alternate angles are equal
Co-interior angles add to 180°

If one angle is 112°, the co-interior partner is:

180 − 112 = 68°

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°”.

Useful next step: cover the worked answer and try the same method on a similar question before moving on.