A+ A− B

Parallel Lines cut by a Transversal

Angles formed

What is a transversal ?

Answer: A transversal is a line, like the red one below, that intersects two other lines.
Typically, the intercepted lines like line a and line bthe above are parallel, but they do not have to be.

What is so special about a transversal ?

Answer: When a transversal cuts (or intersects) parallel lines several pairs of congruent and supplementary angles are formed.
The Congruent Angle Pairs
There are 3 types of angles that are congruent: Alternate Interior, Alternate Exterior and Corresponding Angles.
parallel line transversal angle picture

Can you make a Z?

alternate interior angles draw letter z Some people find it helpful to use the 'Z test ' for alternate interior angles.
If you can draw a Z or a 'backwards z', then the alternate interior angles are the ones that are in the corners of the Z

Can you identify each type of congruent angle in the picture below?
Picture parallel lines cut by transversal

Alternate Interior Angles

Alternate Exterior Angles

Corresponding Angles
Load a Similar Problem?

The Supplementary Angle Pairs
There are 2 types of supplementary angles that are formed: Same side interior and same side exterior
Same Side Interior
same side interior
Same Side Exterior
same side interior
Interactive Parallel Line and Angles

Click and Drag Points E and F to explore the relationship of corresponding, alternate interior and alternate exterior angles that are formed by a transversal and parallel lines. (Full Size Version)

Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player