# Triangles

#### Back to Geometry

The name *triangle* tells us that the shape has three (tri-) angles. Triangles can be given additional names based on two criteria:

- the length of their sides
- and the size of their angles.

Both of these will be covered in this tutorial.

## Classifying based on sides

Triangles that have identical sides are called *equilateral* triangles. Triangles that have only got two identical or equal sides are called *isosceles* triangles. Triangles that have *not* got any identical sides (i.e. all of the sides are unique in length), are called *scalene* triangles. These are all displayed in the image below.

## Classifying based on angles

The naming conventions for triangles based on angles are easier to remember than those above, but only so long as you remember the naming conventions for angles. Triangles that with all angles less than 90 degrees are called *acute* triangles. Triangles that have got one 90 degree angle are called *right-angled* triangles. Triangles that have got one angle that is more than 90 degrees are called *obtuse* triangles.

## Classifying based on BOTH sides and angles

It is possible for a triangle to fall into more than one category. For example, the triangle below has got a right angle, as well as two sides that are of equal length. This means that we can call it both an *isosceles* triangle, and a *right-angled* triangle.

Have a look at the below two triangles and try to work out what two names they could be given, then roll your mouse over the image to see if you've got the right answer.

## Back to Naming Conventions |