Parallel Lines

A unit in almost every geometry class has parallel lines cut by a transversal.

Same-side angles are supplementary (notice the s.s.s.), and the other angles are congruent. We will have a lot more to say about this, as well as some GradeA diagrams to make your life easier!

Two lines that are in the same plane that do not intersect are said to be parallel. (More information about the undefined terms in geometry).

A transversal is a line that intersects two or more lines. When this happens, eight different angles are created, many of which come in special pairs. This becomes particularly interesting whenever the two lines are also parallel because the angles are either congruent or supplementary.

Interior: Between the Parallel Lines	Exterior: Outside the Parallel Lines

Corresponding Angles: There are four pairs of corresponding angles that are created by the transversal. The pairs are in the same location in their respective cluster of four. For example, the "top-left" angle corresponds with the other "top-left" angle. See the diagram below.

Top-Left with Top-Left		Top-Right with Top-Right
Bottom-Left with Bottom-Left		Bottom-Right with Bottom-Right

Alternate Interior Angles: We already know that interior angles are between the two lines. The word alternate means that the angles are on opposite sides of the transveral. See for yourself.

Alternate Exterior Angles: Almost the same as alternate interior, except now the angles are on the outside. See below.

Same-Side Interior Angles: Interior angles that are both on the same side of the transversal. See below.

Same-Side Exterior Angles: Exterior angles that are both on the same side of the transversal. See below.

Return from parallel lines to other free geometry help topics.

GradeA also offers a geometry glossary so that you can learn all geometry vocabulary that you will encounter along the way!