Originally, Louisiana was the territory of Spain and then France, both Roman Catholic countries, and those colonial powers divided their lands according to church parish boundaries for administrative purposes. After annexation by the United States, Louisiana continued to refer to administrative regions as parishes, not counties.
In 1803, the United States more than doubled its existing territorial holdings by purchasing the Louisiana Territory from the French government for $3 million. The American government divided the new territory into smaller divisions, but the legislature governing the territory of Louisiana elected to use the term "parish" for administrative boundaries, even after joining the United States. Throughout its history, Louisiana has retained the term, even though all of the other 49 states refer to their administrative regions as counties.
The entire territory (about 828 million square miles) of the Louisiana Purchase stretched from the Gulf of Mexico and the Mississippi River to the border with Canada and the Rocky Mountains. This region would eventually form some or all of 15 of the 50 states.
The land was purchased for less than three cents per acre, which was considered to be an excellent value for money. This acquisition has long been viewed as one of the major achievements during the presidency of Thomas Jefferson.