Skill: Identify parallel lines

Parallel lines are lines that:

• stay the same distance apart,
• lie in the same flat plane, and
• never meet, even if you extend them forever.

They can be horizontal, vertical, or slanted. They do not have to be the same length.

Example: Look at rectangle $(ABCD)$.

A -------- B
|          |
|          |
D -------- C

Find the pairs of parallel sides.

1. Compare the top side $(AB)$ with the bottom side $(DC)$.
   They stay the same distance apart and will never meet.
   So, $(AB)$ is parallel to $(DC)$.

2. Compare the left side $(AD)$ with the right side $(BC)$.
   They also stay the same distance apart and will never meet.
   So, $(AD)$ is parallel to $(BC)$.

The pairs of parallel sides are:

$$AB \parallel DC$$

and

$$AD \parallel BC$$

To check any pair of lines, imagine extending both lines. If they stay the same distance apart and never meet, they are parallel.