Quadrilateral, the name given in history to the four fortresses of North Italy—Mantua, Verona, Peschiera, and Legnano—which form a sort of outwork to the bastion of the mountains of the Tyrol, and divide the north plain of the Po into two sections by a most powerful barrier. They have figured in all the later wars that have been fought in North Italy, especially in the wars between Austria and the different Italian states.—Russia has a similar combination of four fortresses in Poland, called the Polish Quadrilateral. See NOVOGEORGIEVSK.