# Quadrilateral

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**Quadrilateral**as edited by DavidB4-bot (Talk | contribs) at 18:03, 13 July 2016. This URL is a permanent link to this version of this page.A **quadrilateral** is a polygon with four sides.

As a consequence of the definition, the sum of the angles in a quadrilateral equals 360^{o}, and all quadrilaterals are capable of tessellating an infinite plane.

## Classification of Quadrilaterals

The classification of quadrilaterals is complex, as there are several intersecting sub-categories:

- A (British) trapezium or (American) trapezoid has one pair of opposite sides parallel. A version of this called the isosceles trapezium has the other pair of opposite sides equal
- A parallelogram has two pairs of opposite sides parallel (and equal)
- A rectangle has four right angles
- A rhombus has four equal sides
- A kite has two pairs of adjacent equal sides
- A re-entrant quadrilateral has one internal angle greater than 180
^{o}; this may be contrasted with a convex quadrilateral in which all internal angles are less than 180^{o} - An arrowhead is a re-entrant kite
- A square is both a rectangle and a rhombus