The 6 Main Types of Triangles: by Side and by Angle

Triangles types are named and classified by their sides and by their angles.

By Angle:

• Acute Triangle — All angles are less than 90°.
• Right Triangle — Has an angle that is exactly 90°.
• Obtuse Triangle — Has one angle that is more than 90°.

By Side:

• Equilateral Triangle — All three sides are equal in length.
• Isosceles Triangle — Two sides are equal in length.
• Scalene Triangle — Has no sides that are equal in length.

Special Types of Right Triangles

Within the right triangle, there are 4 special right triangles. The special right triangles by angles are the 45-45-90 Triangle and the 30-60-90 Triangle. The special right triangles by sides are the 3-4-5 Triangle and the 5-12-13 Triangle.