Triangle is a basic geometric shape in two dimensions. There are three sides and three angles. Types of triangle are cansidered by relative lengths of sides and by internal angles. In term of relative lengths of sides, three types. The first type is all sides have the same length. They are called an equilateral triangle. The second type called an isosceles triangle because two sides are equal in length. Third type called a scalene triangle because all sides are unequal. As for internal angles, three types. The first types called a right triangle because it has one of its interior angles measured at 90 degrees. The second type is a triangle with all interior angles measured less than 90 degree. This is an acute triangle. The last type is a triangle with one interior angle measured more than 90 degree. This is an obtuse triangle.