# Definition:Right Angle

## Definition

A right angle is an angle that is equal to half of a straight angle.

### Measurement of Right Angle

The measurement of a right angle is $\dfrac {180 \degrees} 2 = 90 \degrees$ or $\dfrac \pi 2$.

## Perpendicular

In the words of Euclid:

When a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right, and the straight line standing on the other is called a perpendicular to that on which it stands.

In the above diagram, the line $CD$ has been constructed so as to be a perpendicular to the line $AB$.