A quadrilateral is a polygon with four sides.

As a consequence of the definition, the sum of the angles in a quadrilateral equals 360^{0}, 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} - An arrowhead is a re-entrant kite
- A square is both a rectangle and a rhombus