# Definition:Triangle (Geometry)/Right-Angled

## Contents

## Definition

A **right-angled triangle** is a triangle in which one of the vertices is a right angle.

Note that in order to emphasise the nature of the right angle in such a triangle, a small square is usually drawn inside it.

Note the following nomenclature in the context of a **right-angled triangle**:

### Hypotenuse

In a right-angled triangle, the opposite side to the right angle is known as the **hypotenuse**.

In the above figure, the side labeled $b$ is the **hypotenuse**.

### Adjacent

In a right-angled triangle, for a given non-right angled vertex, the adjacent side which is *not* the hypotenuse is referred to as **the adjacent**.

In the above figure, the **adjacent** to vertex $A$ is side $c$.

### Opposite

In a right-angled triangle, for a given non-right angled vertex, the opposite side is referred to as **the opposite**.

In the above figure, the **opposite** to vertex $A$ is side $a$.

### Legs

In a right-angled triangle, the two sides which are not the hypotenuse are referred to as its **legs**.

## Also known as

A **right-angled triangle** is also often referred to as a **right triangle**.

## Euclid's Definition

In the words of Euclid:

*Further, of trilateral figures, a***right-angled triangle**is that which has a right angle, an**obtuse-angled triangle**that which has an obtuse angle, and an**acute-angled triangle**that which has its three angles acute.