Parallel and perpendicular lines
Identify horizontal and vertical lines and pairs of perpendicular and parallel lines
What a learner can do afterwards
- Point out horizontal and vertical lines in shapes and real-world contexts
- Identify a pair of parallel lines (lines that never meet and are always the same distance apart)
- Identify perpendicular lines (lines that meet at a right angle)
1 · Read
Look around you. Some lines lie flat, like the edge of a table. We call these horizontal lines. Other lines stand up tall, like a flagpole or a doorframe. We call these vertical lines.
Sometimes two lines cross and make a right angle, like a square corner. You already know a right angle. When two lines meet like that, we call them perpendicular lines.
Mia watches a train go by. She notices the two rails of the track run side by side. They never touch and always stay the same distance apart. Mia knows these rails are parallel lines.
If two lines make a right-angle corner, they are perpendicular. If two lines run side by side and never get closer or farther apart, they are parallel.
Perpendicular lines cross at a right angle; parallel lines run side by side and never meet.
More to read
Parallel and perpendicular lines review
Parallel lines are lines that never meet. They stay the same distance apart everywhere, like the two long edges of a ruler or a pair of railway tracks. Even if you drew them much longer, they would still never cross. Perpendicular lines are different. They cross each other and make a neat square corner, which is called a right angle. The letter T shows perpendicular lines where the top bar meets the stem.
Two or more lines
When you look at two or more lines together, ask one question first. Do they cross or do they stay apart. If they stay the same distance apart and never meet, they are parallel. The sides of a ladder and the edges of a door show this well. If they cross at a square corner, they are perpendicular. Window frames are full of perpendicular lines, and spotting them gets easier with practice.
Points and lines all around
Lines are all around you, in buildings, playgrounds and books. Horizontal lines lie flat like the horizon, and vertical lines stand straight up like a flag pole. A horizontal line and a vertical line that meet always make a right angle, so they are perpendicular. Try finding three horizontal and three vertical lines in your classroom. Then check which pairs meet at square corners.
3 Ways to Figure out if Two Lines Are Parallel
There are simple ways to test whether two lines are parallel. One way is to measure the gap between them in two places. If the gap is exactly the same both times, the lines are parallel. Another way is to check their slant. Lines that slant in exactly the same way and never get closer are parallel. A third way is to look for a line crossing both of them. If it makes matching corners on each line, the two lines are parallel.
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2 · Watch
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Where it sits
Learn first
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
17 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.