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In U.S. land surveying under the Public Land Survey System (PLSS), a section is an area nominally one square mile, containing 640 acres (260 ha), with 36 sections making up one survey township on a rectangular grid.
The legal description of a tract of land under the PLSS 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 into smaller squares by repeated halving and quartering. A quarter section is 160 acres and a "quarter-quarter section" is 40 acres (16 ha). In 1832 the smallest area of land that could be acquired was reduced to the 40 acre quarter-quarter section, and this size parcel became entrenched in American mythology. After the Civil War, Freedmen (freed slaves) were reckoned to be self-sufficient with "40 acres and a mule." In the 20th century real estate developers preferred working with 40 acre parcels. The phrases "front 40" and "back 40," referring to farm fields, indicate quarter-quarter sections of land.
One of the reasons for creating sections of 640 acres was the ease of dividing into halves and quarters while still maintaining a whole number of acres. A section can be halved seven times in this way, down to a 5 acres (2.0 ha) parcel, or half of a quarter-quarter-quarter section—an easily surveyed 50 square chain area. This system was of great practical value on the American frontier, where surveyors often had a shaky grasp of mathematics and were required to work quickly.
A description of a quarter-quarter section in standard abbreviated form, might look like "NW 1/4, NE 1/4, Sec. 34, T.3S, R.1W, 1st P.M.". In expanded form this would read "the Northwest quarter of the Northeast quarter of Section 34 of Township 3 South, Range 1 West, first Principal Meridian".
The existence of section lines made property descriptions far more straightforward 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 (260 ha) 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.
Sections are (often) broken up into 40 acre blocks, or quarter quarter sections. These are labeled as follows: 
Due to such factors as survey errors, poor instrumentation, difficult terrain, and sloppy work by surveyors, and primarily the curvature of the earth, that is as sections were surveyed from south and east to the north and west the sections got narrower on the north end, being the reason for the errors distributed on the north end of the township, the errors on the west included layout errors also influenced by the compass, or compass based instrument, the primary tool for obtaining direction. it is common for actual sections to differ from the PLSS ideal of one square mile. The distortions and errors were, by design, distributed to the northern and western edges of each township. As a result the sections in these areas diverge the most from the ideal shape and size. In addition there was a need to regularly adjust the entire township grid to account for distortions caused by the curvature of the Earth and the convergence of meridians toward the poles. In places where the grid was corrected, or where two grids based on different principal meridians came together, section shapes are often highly warped. Despite the survey errors and flaws, once the grid was established it remained in force mainly because the monuments of the original survey, when recovered, hold legal precedent over subsequent resurveys.
The Public Land Survey System was not the first to define and implement a survey grid. A number of similar systems were established, often using terms like section and township but not necessarily in the same way. For example, the lands of the Holland Purchase in western New York were surveyed into a township grid before the PLSS was established. In colonial New England land was often divided into squares called towns or townships, and further subdivided into parcels called lots or sections.
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.
Another exception to the usual use of sections and section numbering occurs when most of a parcel, or lot, falls under a body of water. The term "government lot" is used for such parcels and they are usually described separately from the rest of the section using single numbers (such as "Government Lot 5 of Section 15"). Also, parcels within a platted subdivision are often specified by lot number rather than using PLSS descriptions.
Where Spanish Land Grant descriptions occur in Florida that predate PLSS or even the U.S. itself, deviation from typical section numbering and size/shape often takes place. In an effort to honor these Land Grants after the U.S. took control of Florida, surveyors would use descriptions from confirmed Land Grants to establish their initial boundaries and created PLSS sections that extrapolated from those lines. Often, the amount of land left over in areas immediately surrounding the grants was grossly undersized or awkwardly shaped. Those tracts are referred to as "fractional sections" and often are not subject to township or range definitions. An example of such a legal description's beginning would read "Being a portion of Fractional Sec. 59, Township 0 South, Range 0 West".
Also, land north of the Watson Line near the Georgia border was not subject to the standard US section, township and range designations, since the State of Georgia had claimed and laid out counties and surveyed its public lands south to that line into what eventually became part of the State of Florida. The exact location of the Georgia-Florida state line was ultimately confirmed by an Act of Congress, approved April 9th, 1872.