QUADRILATERAL  , in

geometry, a figure enclosed by four straight lines . It is also a military term applied to a combination of four fortresses mutually supporting one another . The fortresses of Namur, Liege, Maastricht, and Louvain, and also those of Silistria, Rustchuk, Shumla, and Varna, were so called . But the most famous quadrilateral was that of the four fortified towns of north Italy—Mantua, Peschiera, Verona, and Legnago, A the two former of which are situated on the Mincio and the two latter on the Adige . The real value of the quadrilateral, which gave Austria such a firm hold on Lombardy, lay in the great natural strength of Mantua and in the readiness with which troops and supplies could be poured into Verona from the north .