# Apex angle (vertex angle) of an isosceles triangle

**An apex angle** is the angle between the lines that define the apex.

Apex angle of an **isosceles triangle. **

In an **isosceles triangle**, the **apex** is the **vertex** where the two sides of equal length meet, opposite the unequal third side

The angles at the base are called the **base angles,** and the angle opposite the base is called the **vertex angle.**

An **isosceles triangle** is a triangle that has two sides of **equal length.**

The two equal sides are called the **legs** and the third side is called the **base** of the triangle.