## Dictionary Definition

The Oxford English Dictionary defines a right triangle as "a triangle containing a right angle."## The Parts of a Right Triangle

- A right triangle has one right angle (an angle of 90°). It is shown by drawing a small square.
- The longest side (opposite the right angle) is called the hypotenuse.
- The other two shorter sides are called legs or catheti (singular cathetus). They are perpendicular to each other.
- If another angle is labelled (such as the angle labelled θ in the image below) then we can name the two shorter sides.
The side next to the angle is called the adjacent. The side opposite the angle is called the opposite.
**Note:**The adjacent and opposite only exist in relation to an angle. The same side can be an adjacent or an opposite depending on which angle they refer to.

## What's in a Name?

A right triangle gets its name from the fact it has a right angle - an angle of 90°. The right triangle is called a right-angled triangle in British English.## The Other Angles in a Right Triangle

The 3 angles in a triangle**must**add to 180°. If one angle is 90°, the other two angles must add to 90°.

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