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Sectioning a township.

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.

The existence of section lines made property descriptions far more straightfoward than the old metes and bounds system. The establishment of standard east-west and north-south lines ("township" and "range lines") meant that deeds could be written without regard to temporary terrain features such as trees, piles of rocks, fences, and the like, and be worded in the style such as "Lying and being in Township 4 North; Range 7 West; and being the northwest quadrant of the southwest quadrant of said section," an exact description in this case of 40 acres, as there are 640 acres in a square mile.

The importance of "sections" was greatly enhanced by the passage of "An Ordinance for ascertaining the mode of disposing of lands in the Western Territory" of 1785 by the U.S. Congress (see Land Ordinance of 1785). This law provided that lands outside the then-existing states could not be sold, otherwise distributed, or opened for settlement prior to being surveyed. The standard way of doing this was to divide the land into sections. An area six sections by six sections would define a township. Within this area, one section was designated as school land. As the entire parcel would not be necessary for the school and its grounds, the balance of it was to be sold, with the monies to go into the construction and upkeep of the school.

In many jurisdictions, roads were run along every section line, giving access to previously remote areas and serving in many instances as firebreaks. In some locales, these lines were designated as the basis for a street numbering system; in Tulsa, Oklahoma, for example it can be correctly assumed that 190th Street is exactly seventeen miles beyond 20th Street, each "block" representing one-tenth of a mile. In Oklahoma City and metropolitan areas of Arizona, most notably Phoenix and Tucson, all major thoroughfares run along section lines, producing a readily identifiable grid.

Every township is divided into 36 sections, each usually one-mile square. Sections are numbered boustrophedonically within townships as follows (north at top):

 6  5  4  3  2  1       
 7  8  9 10 11 12       
18 17 16 15 14 13       
19 20 21 22 23 24       
30 29 28 27 26 25        
31 32 33 34 35 36        

Sections are also used in land descriptions in the portion of northwestern Georgia that was formerly part of the territory of the Cherokee Nation. They are not, however, part of the PLSS and are irregular in shape and size. See Cherokee County, Georgia for more information on the historical reasons for this.

For a detailed description of the method of establishing and subdividing townships see the Canadian Dominion Land Survey article.