The Hanseatic League was a trading association of German merchants and cities that was active from the middle of the 12th century to the middle of the 17th century. At its largest extent in the mid-1400s, league membership included over 100 cities. Hanseatic merchants maintained permanent enclaves called kontors in other major cities in northern and western Europe. At the height of its power, the Hanseatic League held a virtual monopoly on trade in the North and Baltic seas.