The Southern United States is one of the four census regions of the United States as defined by the U.S. Census Bureau. It is between the Atlantic Ocean and the Western United States, with the Midwestern and Northeastern United States to its north and the Gulf of Mexico and Mexico to its south. Historically, the South was defined as all states south of the 18th-century Mason–Dixon line, the Ohio River, and the 36°30′ parallel. Within the South are different subregions such as the Southeast, South Central, Upper South, and Deep South. Maryland, Delaware, Washington, D.C., and Virginia have become more culturally, economically, and politically aligned in certain aspects with the Northeastern United States and are sometimes identified as part of the Northeast or Mid-Atlantic. The U.S. Census Bureau continues to define all four places as formally being in the South.