Plat maps, or plats, tell you who owned a parcel of land at the time the map was published. When many maps are collected in a volume (usually for a specific county), they are called plat books.
If you are researching the history of a building or neighborhood, plats are key resources for you. They give the names of land owners at the time the map was made.
You can also gain insight into land use patterns. Numerous small parcels usually indicate an urban or urbanizing area. Larger, undivided parcels usually indicate agricultural or undeveloped land.
"A document showing the legal divisions of land by lot, street, and block number. A plat map is usually drawn after the property has been described by some other means, such as a government survey. Once a plat map is prepared, property descriptions are defined by referring to the map. Plat maps are usually recorded by a government agency."
From Garner, Bryan. Black’s Law Dictionary, edition 11. Thomson Reuters, 2019.
This detail depicting the area around Liberty, Missouri, from plate 28 of the 1898 Standard Atlas of Clay County, Missouri shows William Jewell College east of town, a brickyard south of town, and a wine vault southwest of town.
The US government established the Public Land Survey System in 1785 to measure lands that would be offered for sale to the public. Each survey map is divided into townships, ranges, and sections. Each location on a plat map can be referenced with a uniform reference numbering system, which is still used today as part of the legal description of a piece of property.
The Bureau of Land Management (BLM), US Department of the Interior, has additional records and documents regarding the first sale of government lands to an individual.
Missouri's land documents are complicated because private individuals received land grants from the Spanish and French colonial governments before the Louisiana Territory, which included what is now the state of Missouri, became part of the United States in 1804. Find more information here.