*This article is about mathematics. For rhomboid muscles in anatomy, see Rhomboid major muscle and Rhomboid minor muscle. For rhomboid in botany see Leaf shape.*

In geometry, a **rhomboid** is a parallelogram in which adjacent sides are of unequal lengths and angles are oblique.

A shape like a circle with sides of equal length (equilateral) is not a rhombus.

A shape like a rhomboid with right angled corners is a rectangle.

The word Rhomboid which means *rhomb*-like was commonly used in the 19th century for a parallelogram which was neither a rectangle nor a rhombus. Today it is more often used for a solid figure with six faces in which each face is a parallelogram and opposite faces in pairs lie in parallel planes. Some crystals are formed in three-dimensional rhomboids. It is also sometimes called a rhombic prism. The term shows up frequently in science terminology referring to both its two and three dimensional meaning.

Euclid introduces 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 uses the definition of rhomboid again and introduces the word parallelogram in Proposition 31 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 .

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