Legs, base, vertex angle, and base angles. In an isosceles triangle, the two equal sides are called legs, and the third side is called the base. The angle formed by the two equal sides is called the vertex angle. The other two angles are called base angles (Figure 1).
Figure 1 Parts of an isosceles triangle.
In a right triangle, the side opposite the right angle is called the hypotenuse, and the other two sides are called legs (Figure 2).