Triangles can be categorized according to the their internal angles, described below using degrees of arc.

• Right Triangle (or right-angled triangle): Possesses one 90° internal angle (a right angle). The side opposite to the right angle is the hypotenuse; it is the longest side in the right triangle. The other two sides are the legs of the triangle.

• Obtuse Triangle: Possesses one internal angle larger than 90° (an obtuse angle).

• Acute Triangle: Possesses internal angles that are all smaller than 90° (three acute angles). An equilateral triangle is an acute triangle, but not all acute triangles are equilateral triangles.