A **transversal** is a line which intersects two or more other lines.

In the image [*above*], lines *m* and *n* are cut by transversal *t*.

When lines intersect, a variety of angles are formed. Many of these angles have names.

In the image above, angles 3 and 6, and angles 4 and 5 are **alternate interior angles**.

When the lines cut by a transversal are parallel lines, alternate interior angles are congruent.

In the image above, angles 1 and 8, and angles 2 and 7 are **alternate exterior angles**.

When the lines cut by a transversal are parallel lines, alternate exterior angles are congruent.

In the image above, the following pairs of angles are **corresponding angles**: 1 and 5, 3 and 7, 2 and 6, and 4 and 8.

When the lines cut by a transversal are parallel lines, corresponding angles are congruent.

In the image above, angles 3 and 5, and angles 4 and 6 are **same side interior angles**.

When the lines cut by a transversal are parallel lines, same side interior angles are supplementary.
In the image above, the following pairs of angles are **vertical angles**: 1 and 4, 2 and 3, 5 and 8, and 6 and 7. Vertical angles are congruent.