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A transversal is a line that crosses two other lines. In both diagrams, the line PQ is a transversal to both the lines AB and CD.

Two diagrams. Both have a pair of lines AB and CD cut by a transversal PQ. Left diagram, AB is parallel to CD

Corresponding angles

In each of the diagrams below, the two marked angles are called corresponding angles because they are in corresponding positions around the two vertices M and N.

Four diagrams. All have a pair of lines cut by a transversal.
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Corresponding angles and parallel lines

If the lines AB and CD are parallel, then the corresponding angles \(\angle\)DGF and \(\angle\)BHF, marked \(\alpha\) and β, are equal.

Pair of parallel lines AB and CD cut by transversal EF intersecting at H and G.
Detailed description

Example 1

Find θ in the diagram.

Pair of parallel lines AB and CD cut by transversal. One angle marked 121 degrees and its corresponding angle marked theta.


θ = 121° (corresponding angles AB || CD)