Several types of triangles exist, including scalene, isosceles, equilateral, right, obtuse and acute. Triangles are categorized according to their sides, angles or a combination of both.

Scalene, isosceles and equilateral triangles are categorized by their sides. Scalene triangles are triangles with no sides that are equal in length. Isosceles triangles have at least two equal sides. Equilateral triangles have three equal sides. Each of the three angles in an equilateral triangle measures 60 degrees.

Right, obtuse and acute triangles are classified according to their angles. Right triangles have a 90-degree angle. Obtuse triangles have one angle that is greater than 90 degrees. Acute triangles have all angles less than 90 degrees, and their angles are referred to as acute angles.

Triangles also can be classified by both their sides and angles. A right isosceles triangle has one right angle and two sides that are equal. An acute scalene triangle is a triangle with all acute angles and no equal sides. An obtuse isosceles triangle is a triangle with two equal sides and one obtuse angle. The sum of all the angles in any kind of triangle is 180 degrees. The third side of any kind of triangle must always be shorter than the sum of the other two sides.