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A polygon that has four sides and four vertices (corners). Quadrilaterals, and polygons in general, may be convex or concave. A convex quadrilateral may be further classified as a trapezoid or a British trapezium (one pair of opposite sides are parallel), a trapezium (no sides parallel); an isosceles trapezoid (U.S.) or an isosceles trapezium (U.K.) (two of the opposite sides parallel, the two other sides equal, and the two ends of each parallel side of equal angles); a parallelogram (opposite sides are parallel); a kite (two adjacent sides of equal length, the other two sides of equal length); a rhombus (four sides of equal length); a rectangle (each angle is a right angle); or a square (four sides of equal length, each angle a right angle). Quadrangular prisms and quadrangular pyramids are ones whose bases are quadrilateral.

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