# Definition:Circle/Chord

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## Definition

A **chord** of a circle is a straight line segment whose endpoints are on the circumference of the circle.

In the diagram above, the lines $CD$ and $EF$ are both **chords**.

Thus a **chord** is a line segment joining the endpoints of an arc of the circle.

Note that under this definition, the diameter itself is in fact a **chord**.

In the words of Euclid:

*A straight line is said to be***fitted into a circle**when its extremities are on the circumference of the circle.

(*The Elements*: Book $\text{IV}$: Definition $7$)

## Sources

- 2008: Ian Stewart:
*Taming the Infinite*... (previous) ... (next): Chapter $5$: Eternal Triangles: The origins of trigonometry