In U.S. land surveying, a section is an area nominally one mile square, containing 640 acres (2.6 km²). Nominally, 36 sections make up a survey township on a rectangular grid. As the townships are based on meridians (of longitude) which converge towards the north pole, some sections which vary slightly in size are necessary to compensate. These unusually sized sections generally occur at the northern- or western-most edges of townships.
The legal description of a tract of land in the parts of the United States that use this system includes the name of the state, name of the county, township number, range number, section number, and portion of a section. Sections are customarily surveyed in halves and quarters, and further subdivision in halves and quarters is common. A quarter quarter section is 40 acres (about 162,000 m²), and is the smallest unit of agricultural land commonly surveyed. The phrases "front 40" and "back 40," referring to fields of crops on a farm, refer to quarter quarter sections.
