Parallel Lines, and Pairs of Angles

Parallel Lines

Lines are parallel if they are always the same distance apart (called "equidistant"), and will never meet. Just remember:

The red line is parallel to the blue line in both these cases:


Example 1		Example 2

Parallel lines also point in the same direction.

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.

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Some of those special pairs of angles can be used to test if lines really are parallel:

If Any Pair Of ...	Example:

Corresponding Angles are equal, or	a = e
Alternate Interior Angles are equal, or	c = f
Alternate Exterior Angles are equal, or	b = g
Consecutive Interior Angles add up to 180°	d + f = 180°

... then the lines 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