Parallel Lines, and Pairs of Angles
Lines are parallel if they are always the same distance apart (called "equidistant"), and will never meet. Just remember:
Always the same distance apart and never touching.
The red line is parallel to the blue line in both these cases:
Parallel lines also point in the same direction.
Pairs of Angles
When parallel lines get crossed by another line (which is called a Transversal), 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.
Click on each name to see it highlighted:
Testing for Parallel LinesSome of those special pairs of angles can be used to test if lines really are parallel:
|These lines are parallel, because a pair of Corresponding Angles are equal.|
|These lines are not parallel, because a pair of Consecutive Interior Angles do not add up to 180° (81° + 101° =182°)|
|These lines are parallel, because a pair of Alternate Interior Angles are equal|