Survey township, sometimes called Congressional township, as used by the United States Public Land Survey System, refers to a square unit of land, that is nominally six (U.S. Survey) miles (~9.7 km) on a side. Each 36-square-mile (~93 km2) township is divided into 36 one-square-mile (~2.6 km2) sections, that can be further subdivided for sale, and each section covers a nominal 640 acres (2.6 km2). The townships are referenced by a numbering system that locates the township in relation to a principal meridian (north-south) and a base line (east-west). For example, Township 2 North, Range 4 East is the 4th township east of the principal meridian and the 2nd township north of the base line. Township (exterior) lines were originally surveyed and platted by the US General Land Office using contracted private survey crews. Later survey crews subdivided the townships into sections (interior) lines. Virtually all lands covered by this system were sold according to these boundaries. They are marked on the U.S. Geological Survey topographic maps.
Prior to standardization, some of the Ohio Lands (the United States Military District, the Firelands and the Connecticut Western Reserve) were surveyed into townships of 5 miles (8.0 km) on each side. These are often known as Congressional Townships.
Sections are divided into quarter-sections of 160 acres (65 ha) each and quarter-quarter sections of 40 acres (16 ha) each. In the Homestead Act of 1862, one quarter-section of land was the amount allocated to each settler. Stemming from this are the idiomatic expressions, "the lower 40", which is the 40 acres on a settler's land that is lowest in elevation, in the direction towards which water drains toward a stream, and the "back forty", the portion farthest from the settler's dwelling.
Survey townships are distinct from civil townships. A survey township is used to establish boundaries for land ownership. A civil township is a form of local government. In states with civil townships, the boundaries of survey townships are often coterminous with civil townships. County lines, especially in western States, usually follow township lines, leading to the large number of rectangular counties in the West, which are agglomerations of survey townships.
In western Canada, the Dominion Land Survey adopted a similar format for survey townships, which do not form administrative units. These townships also have the area of 36 square miles (six miles by six miles).