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In this lesson, you will learn the definition and properties of a straight line. You will be given plenty of examples to ensure your understanding of straight lines.

Following the lesson will be a brief quiz to test your new knowledge.

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What is a Straight Line?

You’ve probably drawn all kinds of lines throughout your life. But, what is a line? A line is simply an object in geometry that is characterized as a straight, thin, one-dimensional, zero width object that extends on both sides to infinity. A straight line is essentially just a line with no curves.

Most of the time, when we speak about lines, we are talking about straight lines! Here are some examples of straight lines.

Curved lines are what you get when you use the squiggle line in your computer Paint program. They are usually called a curve or arc and are not straight lines.

More Straight Lines

There are other kinds of straight lines that are helpful to learn.Vertical straight lines go up and down. Horizontal straight lines go from left to right or vice versa.

Parallel straight lines have the same slope and are the same distance apart, so they will never intersect. Perpendicular straight lines cross each other and form four perfect right angles in the process. Slanted or oblique straight lines are just as they sound: they are straight lines at an angle.

Measurements of a Straight Line

Did you know that a straight line makes its own unique angle? And, did you know that the angle has a specific degree measurement that is always the same? If you drew an angle arc from one point of a straight line to another point, it would form a 180 degree angle.

How to Name a Straight Line

You can name a straight line in two ways:You can take any two points on that line and use those points as the name.

You should draw a line above those two points with arrows on either side, indicating that it is a line that extends on either side into infinity.Or, you can use just one letter. Usually this letter is indicated in lower case. A commonly used letter to name lines is y.

This manner of naming a line is usually used when the line does not have any known points on it to reference.

Line Segments and Rays

Sometimes a straight line is cut off by endpoints.

It then forms line segments and rays. Line segments are different from straight lines because they have two defined endpoints. Lines, on the other hand, extend out to infinity. Rays are a mix between a straight line and a line segment. It extends to infinity in one direction and has an endpoint at the other end.

Lesson Summary

To review, a line is characterized as a straight, thin, one-dimensional, zero width object that extends on both sides to infinity.

Although it is strange to think that a straight line would make an angle, remember that a straight line makes a 180 degree angle when you draw an angle arc from one point of a straight line to another point of that straight line.Naming a straight line is easy. You can either use two points on that line for the name or use one letter that identifies the line, which is usually in lower case. Remember that the best way to name a line is by using two points on the line because it is more specific than using one letter.

It’s also important to know the difference between a straight line, a ray, and a line segment. If a straight line is cut off by endpoints on both ends, it’s a line segment. And, if it is just cut off on one end and extends to infinity on the other end, it is a ray.

Straight Line Traits

Straight lines
  • Straight, one dimensional and has zero width
  • Both ends extend to infinity
  • Makes a 180-degree angle when drawing an angle arc from one point to another
  • Can be horizontal, slanted or vertical

Learning Outcome

When you are done, you should be able to identify the qualities of straight lines and distinguish them from rays and line segments.

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