Parallel Lines & Transversals

A transversal is a line that intersects two or more other lines. When it intersects parallel lines, many angles are congruent. Let's take a peek at what this means. Lines k and j are parallel. Line l is a transversal.

As we mentioned before, when this happens we get a bunch of pairs of congruent angles. These pairs have nifty vocabulary terms to go with them. Here they are:

• Corresponding angles - angles that are in the same position on each line. There are four sets of these angles: .
• Alternate interior angles - angles on the opposite sides of the transversal and on the interior of the parallel lines. There are two sets of alternate interior angles: .
• Alternate exterior angles - angles on opposite sides of the transversal and on the exterior of the parallel lines. There are two sets of alternate exterior angles: .

Look Out: these pairs of angles are congruent only when the transversal cuts parallel lines.