Actually, there isn’t one!
No incorporated towns or cities lie within Arlington’s boundaries.
“Arlington is an urban county of about 26 square miles located directly across the
Potomac River from Washington DC. No incorporated towns or cities lie within Arlington’s boundaries. Originally part of the area surveyed for the nation’s capital, the portion on the west bank of the Potomac River was returned to the Commonwealth of Virginia by the U.S. Congress in 1846. This area was known as Alexandria City and Alexandria County until 1920 when the county portion was renamed Arlington County. ” From Arlington County Profile 2009

The History
“In 1791 President George Washington determined that the survey to establish the boundaries of the ten-mile-square Federal District should begin at Jones Point, south of Alexandria, and should proceed northwestward so as to exclude the Falls Church. The survey was made that summer, and boundary stones were set up at one-mile intervals. The District of Columbia was not organized until 1801, however. The part of Fairfax County ceded by Virginia to federal jurisdiction was then organized as Alexandria County, which included the Town of Alexandria. All of the present Arlington County was located in the original Ten Mile Square of the District of Columbia.” From the Arlington Historical Society

What’s in a name?
In 1920 to avoid confusion between Alexandria County and the City of Alexandria and to honor Robert E. Lee, the name of the County was changed to Arlington.

An interesting map at the Library of Congress
Detailed map of part of Virginia from Alexandria to the Potomac River above Washington, D.C. 186-
Shows parts of Arlington and Fairfax counties, part of the defenses of Washington, roads, railroads, towns, vegetation, houses, fences, names of residents, and drainage.

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