What is a Chord in Math?

Instructor: Katharine Reilly
In this lesson, you will learn what a chord of a circle is in math. You will also learn other basic parts of a circle, including the diameter and radius. Finally, you will see which of these parts is a chord and which is not.

What Is a Chord?

Draw a circle on a sheet of paper. Now, mark any two points on the edge of the circle.

circle with two points

Draw a straight line between these two points on the edge of the circle. This line is called a chord.

circle and chord

You have a cookie that you have to cut into two parts to share with your brother. If you cut the cookie with a straight line from one edge of the cookie to another, that line is a chord.

This cut on the side is a chord
cookie with chord

It doesn't matter if one piece is huge and the other piece is tiny, or if they are exactly equal; the line you have cut is a chord.

This cut down the middle is also a chord
cookie with diameter

The Special Chord

You know that a chord can touch any two points on the circle. But, did you know that the diameter of a circle is also a chord? It touches two points on the circle's edge and it also goes through the center point of the circle-- so it is a special chord! It's also the longest chord on a circle.


The diameter also divides the circle into two equal parts. Remember the cookie above? If you cut it through the cookie's center point, then the cookie is divided into two equal parts...and your brother will think that is very fair!

Do you know what else? The pizza you order in a restaurant is measured by its diameter! Chords are very useful parts of a circle.

A 12 inch pizza is measured by its diameter
pizza with diameter

Not a Chord

The radius of a circle touches the center point of a circle and only one point on the circle's edge. It does not have two points that touch the edge of the circle. So, a radius cannot be a chord.

circle with radius

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