Traditional definitions of the Midwest include the Northwest Ordinance "Old Northwest" states and many states that were part of the Louisiana Purchase. The states of the Old Northwest are also known as Great Lakes states and are east-north central in the United States. The Ohio River runs along the southeastern section while the Mississippi River runs north to south near the center. Many of the Louisiana Purchase states in the west-north central United States, are also known as the Great Plains states, where the Missouri River is a major waterway joining with the Mississippi. The Midwest lies north of the 36°30′ parallel that the 1820 Missouri Compromise established as the dividing line between future slave and non-slave states.