# Definition:Quadrilateral/Rhombus

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## Definition

A **rhombus** is a parallelogram whose sides are all the same length.

Its angles *may or may not* all be equal.

## Also known as

A **rhombus** is also known as a **rhomb**.

Particularly when it is oriented so that the longer diagonal is a vertical line, a **rhombus** is often referred to as a **diamond**.

## Also see

- A square is a
**rhombus**all of whose angles are equal to a right angle.

## Euclid's Definitions

In the words of Euclid:

*Of quadrilateral figures, a***square**is that which is both equilateral and right-angled; an**oblong**that which is right-angled but not equilateral; a**rhombus**that which is equilateral but not right-angled; and a**rhomboid**that which has its opposite sides equal to one another but is neither equilateral nor right-angled. And let quadrilaterals other than these be called**trapezia**.

(*The Elements*: Book $\text{I}$: Definition $22$)

## Sources

- 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**rhombus (rhombi)**