# Quadrilateral

A quadrilateral is a polygon with four sides. Special cases of a quadrilateral are: (1) A trapezium—A quadrilateral
with no pairs of opposite sides
**parallel**. (Figure A) (2) A trapezoid—A quadrilateral with one pair of sides parallel. (Figure B) (3) A parallelogram—A quadrilateral with two pairs of sides parallel. (Figure C) (4) A rectangle—A **parallelogram** with all angles right angles. (Figure D) (5) A square—A **rectangle** having all sides of the same length. (Figure E) A complete quadrilateral is a **plane** figure in **projective geometry** consisting of lines *a,b,c,* and *d* (no two of them concurrent) and their points of intersection. (Figure F)

See also Polygons.

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