Pairs of Angles
When parallel lines get crossed by another line (which is called aTransversal), you can see that many angles are the same, as in this example:
These angles can be made into pairs of angles which have special names.
Testing for Parallel Lines
Some of those special pairs of angles can be used to test if lines really are parallel:
|These lines are parallel, because a pair ofCorresponding Angles are equal.|
|These lines are not parallel, because a pair ofConsecutive Interior Anglesdo not add up to 180° (81° + 101° =182°)|
|These lines are parallel, because a pair of Alternate Interior Angles are equal|