The Public Land Survey System (PLSS) is a way of subdividing and describing land in the United States. The PLSS typically divides land into six-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.
The PLSS actually consists of a series of separate surveys. Most PLSS surveys begin at an initial point, and townships are surveyed north, south, east, and west from that point. The north-south line that runs through the initial point is a true meridian and is called the Principal Meridian. The east-west line that runs through the initial point, perpendicular to the Principal Meridian, is called a base line. The southern boundary of Allegan County corresponds to the Michigan Baseline, today reflected in the name of Baseline Road and Base Line Lake.
Each township is identified with a township and range designation. Township designations indicate the location north or south of the baseline, and range designations indicate the location east or west of the Principal Meridian. For example, a township might be identified as Township 7 North (or T.7N.), Range 2 West (or R.2W.), which would mean that it was in the 7th tier of townships north of a baseline, and in the 2nd column of townships west of a baseline. A legal land description of a section includes the State, Principal Meridian name, Township and Range designations with directions, and the section number: Michigan, Michigan Principal Meridian T7N, R2W, Sec 5.
(Text adapted from http://www.nationalatlas.gov/articles/boundaries/a_plss.html; Graphic from Manual of Surveying Instructions, 1973.)