# What Is a Right Angle? (Mathematics Lesson)

# What Is a Right Angle?

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.

# 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

# Complementary Angles

Two angles are complementary if they add up to a right angle.Read more about complementary angles

##### Curriculum

##### Interactive Test

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##### Note

# 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*.