What Are the Three Sides of a Triangle Called?

By Staff WriterLast Updated Mar 31, 2020 6:24:49 AM ET

Although, in general, triangles do not have special names for their sides, in right triangles, the sides are called the hypotenuse, the opposite side and the adjacent side. The names change depending on which angle is the focus angle.


A right triangle is a triangle with one 90-degree angle. The side opposite the 90-degree angle is always called the hypotenuse. When completing geometry or trigonometry problems, a focus angle other than the 90-degree angle is selected. The side opposite that angle is referred to as the opposite side, and the side that completes that angle is called the adjacent side. The ratios between the sides of a right triangle are used in trigonometric functions.