# What Is an Isosceles Triangle?

## What Is an Isosceles Triangle?

An isosceles triangle is a triangle with two equal sides and two equal angles across from them. Note: The lines through the two sides of the triangle in the image above indicate they are of equal length. The two arcs indicate the two angles are equal.

### Dictionary Definition

The Merriam-Webster dictionary defines an isosceles triangle as "a triangle in which two sides have the same length."

## Real Examples of Isosceles Triangles

Below are real examples of isosceles triangles. In each case

• 2 sides are the same length.

• 2 angles are the same.

## The Parts of an Isosceles Triangle

An isosceles triangle has 2 equal sides. These sides are called legs. The third side is called the base. An isosceles triangle has 2 equal angles. These are the angles between each leg and the base. They are called base angles. The angle between the legs is called the vertex angle. ## Properties of Isosceles Triangles

### The Symmetry of Isosceles Triangles

An isosceles triangle has 1 line of symmetry. It is the line that goes through the vertex angle and the midpoint of the base. ## See Also

What is a triangle? What is an angle? Interior angles of a triangle