The Lesson

A right angle is an angle of 90°. A right angle is a quarter of a revolution.

right angle 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:

examples of right angles. square in angle and 90 degrees

A Real Example of a Right Angle: The Right Triangle

A right triangle is a triangle containing a right angle.

right angle in a right triangle 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".

straight line bisected gives equal right angles Lines that are at a right angle to each other are called perpendicular.

Complementary Angles

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

angles that add up to 90 degrees are complementary angles