What is a Straight Line? - Definition & Examples

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  • 0:00 What is a Straight Line?
  • 0:36 More Straight Lines
  • 1:04 Measurements of a…
  • 1:20 How to Name a Straight Line
  • 1:48 Line Segments and Rays
  • 2:11 Lesson Summary
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Lesson Transcript
Instructor: Karin Gonzalez

Karin has taught middle and high school Health and has a master's degree in social work.

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.

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.

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