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An oblong is a quadrilateral whose angles are all right angles, but whose sides are not all the same length:


Also known as

The word oblong is rarely seen nowadays.

Rectangle is the term usually used instead, unless it is essential to emphasise that such a quadrilateral is not a square.

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 $22$)