# How I wish I'd taught... shape [The Triangle]

Triangle: literally means three (tri) angles and so has 3 **interior angles**. But triangles are very special due to the wide variety of types of their properties.

Triangles have interior angles which sum to 180Â°. But why?

__Acute triangles__

An acute triangle has interior angles that are all acute (<90Â°).

__Right angle__

A right angle triangle has one angle equal to 90Â°

__Obtuse triangles__

An obtuse triangle has one obtuse angle.

__Equilateral triangles__

An equilateral triangle has three sides of equal length, and therefore all three interior angles are equal 60Â°.

__Isosceles triangle__

The word **isosceles** literally translates as '**equal legs**'. An isosceles triangle is a triangle with at least two equal side lengths and to equal angles.

__Isosceles right angle__

Like an isosceles triangle but with a right angle. Two sides remain the same length and two angles are equivalent.

__Scalene__

A scalene triangle has unequal sides and angles.