Quadrilateral

Quadrilateral, in geometry, plane 4-sided polygon. Quadrilaterals with 2 pairs of sides parallel are called parallelograms; with one pair of sides parallel, trapezoids; with no 2 sides parallel, trapeziums (the word trapezium is often used as a synonym of trapezoid). Parallelograms whose sides are all of equal length are termed rhombuses. Each side of a parallelogram is equal in length to the side parallel to it, and each interior angle is equal to the interior angle diametrically opposite it. A parallelogram whose interior angles are each 90° is a rectangle; a special case of this is the square, all of whose sides are equal. The sum of the interior angles of a quadrilateral is always 360°.