The Lesson

There are different types of triangles. Triangles can be classified by:

Type of Triangle by Lengths of Sides

Triangles are a different type depending on how many sides are the same length (which is also how many angles are equal). Triangles are equilateral, isosceles or scalene depending on whether 3, 2 or 0 side lengths and angles are equal.

Equilateral Triangles

Equilateral triangles have 3 equal side lengths and angles.
3 sides are equal lengths. 3 angles are equal (60°).

Isosceles Triangles

Isosceles triangles have 2 equal side lengths and angles.
2 sides are equal lengths. 2 angles are equal.

Scalene Triangles

Scalene triangles have 0 equal side lengths and angles.
0 sides are equal lengths. 0 angles are equal.

Interactive Game on the Types of Triangle

Here is an interactive game to help you learn about the types of triangle.

Type of Triangle by Angles

Triangles are a different type depending on the size of the angles inside the triangle. Triangles are acute, obtuse or right triangles depending on whether all angles are acute (less than 90°), one angle is obtuse (more than 90°) or one angle is a right angle (90°).

Acute Triangles

Acute triangles have all acute angles (less than 90°).
3 angles are less than 90°.

Obtuse Triangles

Obtuse triangles have one angle that is obtuse (more than 90°, less than 180°).
1 angle is more than 90°. 2 angles are less than 90°.
Right triangles have one right angle (90°).
1 angle is 90°. 2 angles are less than 90°.

Lesson Slides

Triangles can be classified by both lengths of sides and by angles. This means a triangle can be both acute and isosceles, or obtuse and scalene. The slider below shows more real examples of the types of triangles.

Interactive Widget

Here is an interactive widget to help you learn about the types of triangles.

What Is a Triangle?

A triangle is a 2-dimensional shape with three sides and three angles.

Indicating Equal Sides

If a triangle has sides of equal length, the equal sides are indicated by drawing a short line through the sides:

Indicating Equal Angles

Angles are indicated by drawing an arc between the sides. Equal angles can be indicated by drawing the same number of arcs at each angle. The two angles with a two arcs are equal to each other:

The Interior Angles of a Triangle

The angles between the sides, that are inside the triangle, are called interior angles.

The interior angles of the triangle determine whether a triangle is acute, obtuse or right.

Oblique Triangles

A triangle without a right angle is called an oblique triangle. Acute triangles and obtuse triangles are both oblique triangles.