# Difference between revisions of "Quadrilateral"

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* A [[rhombus]] has four equal sides | * A [[rhombus]] has four equal sides | ||

* A [[kite]] has two pairs of adjacent equal sides | * A [[kite]] has two pairs of adjacent equal sides | ||

− | * A [[re-entrant quadrilateral]] has one internal angle greater than 180<sup>0</sup> | + | * A [[re-entrant quadrilateral]] has one internal angle greater than 180<sup>0</sup>; this may be contrasted with a [[convex quadrilateral]] in which all internal angles are less than 180<sup>0</sup> |

* An [[arrowhead]] is a re-entrant [[kite]] | * An [[arrowhead]] is a re-entrant [[kite]] | ||

* A [[square]] is both a [[rectangle]] and a [[rhombus]] | * A [[square]] is both a [[rectangle]] and a [[rhombus]] |

## Revision as of 12:31, 14 May 2007

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
^{0}; this may be contrasted with a convex quadrilateral in which all internal angles are less than 180^{0} - An arrowhead is a re-entrant kite
- A square is both a rectangle and a rhombus