# 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.

# Complementary Angles

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

Geometry Lessons
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# 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.

# OTHER TYPES OF ANGLES

Right angles are one type of angle, the other types are acute, obtuse, straight, reflex and full.