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Traditionally, in two-dimensional geometry, a **rhomboid** is a parallelogram in which adjacent sides are of unequal lengths and angles are non-right angled.

Rhomboid | |
---|---|

Type | quadrilateral, trapezium |

Edges and vertices | 4 |

Symmetry group | C_{2}, [2]^{+}, |

Area | b × h (base × height);ab sin θ (product of adjacent sides and sine of the vertex angle determined by them) |

Properties | convex |

The terms "rhomboid" and "parallelogram" are often erroneously conflated with each other (i.e, when most people refer to a "parallelogram" they almost always mean a rhomboid, a specific subtype of parallelogram); however, while all rhomboids are parallelograms, not all parallelograms are rhomboids.

A parallelogram with sides of equal length (equilateral) is called a *rhombus* but not a rhomboid.
A parallelogram with right angled corners is a *rectangle* but not a rhomboid.

Euclid introduced the term in his *Elements* in Book I, Definition 22,

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 and angles equal to one another but is neither equilateral nor right-angled. And let quadrilaterals other than these be called trapezia.— Translation from the page of D.E. Joyce, Dept. Math. & Comp. Sci., Clark University [1]

Euclid never used the definition of rhomboid again and introduced the word parallelogram in Proposition 34 of Book I; *"In parallelogrammic areas the opposite sides and angles are equal to one another, and the diameter bisects the areas."* Heath suggests that rhomboid was an older term already in use.

The rhomboid has no line of symmetry, but it has rotational symmetry of order 2.

In biology, rhomboid may describe a geometric rhomboid (e.g. the rhomboid muscles) or a bilaterally-symmetrical kite-shaped or diamond-shaped outline, as in leaves or cephalopod fins.^{[1]}

In a type of arthritis called pseudogout, crystals of calcium pyrophosphate dihydrate accumulate in the joint, causing inflammation. Aspiration of the joint fluid reveals rhomboid-shaped crystals under a microscope.

In anatomy, rhomboid-shaped muscles include the rhomboid major muscle and the rhomboid minor muscle.

**^**"Decapodiform Fin Shapes".

- Weisstein, Eric W. "Rhomboid".
*MathWorld*.