## The Lesson

A right angle is an angle of 90°. A right angle is a quarter of a revolution.**Note**: Right angles are often denoted by adding a small right angle that forms a square with the angle.

## Dictionary Definition

The Oxford English Dictionary defines a right angle as "an angle of ninety degrees; specifically one formed by two perpendicular lines, as in the corner of a square."## Real Examples of Right Angles

The angles below are all right angles:## A Real Example of a Right Angle: The Right Triangle

A right triangle is a triangle containing a right angle.The right triangle is one of the most useful triangles in mathematics. It is used for Pythagoras' theorem and in trigonometry. Read more about right triangles

## Euclid and Right Angles

Euclid defines angles in his book,*Elements*.

**Definition 10:**"When a straight line standing on a straight line makes the adjacent angles equal to one another, each of the equal angles is right".

Lines that are at a right angle to each other are called

*perpendicular*.