Quadrilateral

A quadrilateral is a polygon with four sides.

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

## Classification of Quadrilaterals[edit]

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