# Definition:Right Angle/Perpendicular/Plane

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## Definition

In the words of Euclid:

*A***straight line**is**at right angles to a plane**when it makes right angles with all the straight lines which meet it and are in the plane.

(*The Elements*: Book $\text{XI}$: Definition $3$)

In the above diagram, the line $AB$ has been constructed so as to be a **perpendicular** to the plane containing the straight lines $CD$ and $EF$.

## Also known as

The word **normal** is often used for **perpendicular**, particularly in the context of **vector analysis**.

Also, in the context of **linear algebra** and **analysis**, the word **orthogonal** is often encountered, which is a generalization of the concept of **perpendicularity**, but in a more abstract context than **geometry**