The Public Land Survey System (commonly known as the "Township-Section-Range" system) is a rectangular system of surveys regulated by the U.S. Bureau of Land Management. The PLSS typically divides land into 6-mile-square townships. Townships are subdivided into 36 one-mile- square sections. Sections can be further subdivided into quarter sections, quarter-quarter sections, or irregular government lots. Normally, a permanent monument, or marker, is placed at each section corner. Monuments are also placed at quarter-section corners and at other important points, such as the corners of government lots. Today permanent monuments are usually inscribed tablets set on iron rods or in concrete. The original PLSS surveys were often marked by wooden stakes or posts, marked trees, pits, or piles of rock, or other less-permanent markers.
For more information on the PLSS, visit
http://nationalatlas.gov/articles/boundaries/a_plss.html.