There are six **types of triangles** based on the lengths of their sides and their angles. There are three triangles classified according to the length of their sides: equilateral, isosceles, and scalene. There are three triangles classified according to their angles: right, acute, and obtuse. A triangle belongs to only one category from each group. For example, a triangle can be acute and scalene, or right and isosceles.

### Quick Triangle Review

To review, a triangle is a two-dimensional polygon with three sides, three vertices, and three interior angles. The sum of the three angles always adds up to 180°.

### Three Types of Triangles Based on Sides

**Equilateral triangle**: All three sides are the same length and all interior angles are 60°. This make an equilateral triangle a type of acute triangle.**Isosceles triangle**: Two sides of an isosceles triangle are the same length. The angles are the same at the base of both sides.**Scalene triangle**: None of the sides or interior angles of a scalene triangle are the same.

### Three Types of Triangles Based on Angles

**Right triangle**: One of the angles of a right triangle is 90° or a right angle. Because all three interior angles add up to 180°, the other two angles are acute. The long side of a right triangle is its hypotenuse.**Acute triangle**: All of the angles of an acute triangle are less than 90°.**Obtuse triangle**: One angle of an obtuse triangle is greater than 90°.