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.