The vertex (plural: vertices) is a corner of the triangle. Every triangle has three vertices.

Base

The base of a triangle can be any one of the three sides, usually the one drawn at the bottom. You can pick any side you like to be the base. The base is commonly used to calculate the area of the triangle. In an isosceles triangle, the base is usually taken to be the unequal side.

Altitude

The altitude of a triangle is the perpendicular from the base to the opposite vertex. (The base may need to be extended). Since there are three possible bases, there are also three possible altitudes.

Interior Angles

The three angles on the inside of a triangle at each vertex. The interior angles of a triangle Always add up to 180 degrees. Because the interior angles always add to 180 degrees, every angle must be less than 180 degrees. Therefore, only one of the angles can be 90 degrees or more.

Exterior Angles

The exterior angles of a triangle always add up to 360 degrees.