A right triangle is a triangle where one of the three angles is a perpendicular angle. There are three sides of a right triangle: the adjacent, opposite and hypotenuse sides. The sides are labeled relative to the angle of the right triangle denoted with the symbol (theta). Since one of the three angles of the triangle will always be a right angle, only one angle is needed to describe the shape of the right triangle.

A perpendicular angle is equal to a quarter of a full rotation.

An angle is defined as the amount of rotation between two rays. The point where the rays start is called the vertex.