Alexandria County, Virginia
Alexandria County was part of the original 100 square miles (260 km2) that created the District of Columbia in 1791 pursuant to Article I, Section 8, paragraph 17, of the United States Constitution. The portion of the District created from territory ceded by Virginia in Fairfax County was termed Alexandria County of the District of Columbia. It included all of the present Arlington County, Virginia, plus part of what is now the independent city of Alexandria, Virginia. The area was retroceded to Virginia by an act of the United States Congress on July 9, 1846, following a referendum of its citizens. From 1846 to 1920, the county was known as Alexandria County, Virginia.
In 1870, the independent City of Alexandria seceded from Alexandria County, and because of the confusion between the city and the county having the same name, a movement started to rename Alexandria County. In 1920, the name Arlington County was adopted, after Arlington House, the home of the American Civil War general Robert E. Lee, which stands on the grounds of what is now Arlington National Cemetery. The Town of Potomac was incorporated as a town in Alexandria County in 1908. The town was annexed by the independent city of Alexandria in 1930.
Arlington County shares with a portion of the independent City of Alexandria (including the former town of Potomac) the distinction of being once in Virginia, then ceded to the U.S. government to form the District of Columbia, and later retroceded to Virginia.