Types of Triangles

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°.