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In geometry, a quadrilateral is a polygon with four sides and four vertices. Quadrilaterals, and polygons in general, are broadly divided into two groups: convex and concave polygons. Convex quadrilaterals are further classified as follows:

Quadrilateral is also the name of a group of fortresses in Italy and of the southern part of Dobrudja, ceded by Romania to Bulgaria in the opening month of World War II (see Romania during World War II).