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

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.

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