A rectangular system of land survey divides a district into 24-square mile quadrangles from the meridian (north-south line) and the baseline (east-west line). The tracts are divided into 6-mile-square parts called townships, which are in turn divided into 36 tracts, each 1 mile square, called sections. The rectangular survey system was specified by Congress in 1785 to mark large tracts of land that the United States received in its early years, including the Northwest Territory. The rectangular survey system specifies locations by using a rectangular coordinate system. It consists of principal meridians that run north and south and indicate longitude, and base lines that run east and west and indicate latitude. The quadrangles are further subdivided in 16 townships, with each side of a township measuring 6 miles and covering an area of 36 square miles. Townships are further subdivided into 36 1-square mile sections, with each section equal to 640 acres. In many states, the rectangular survey system is supplemented with a mete-and-bounds description to describe small parcels of land.