Arlington County is located in northeast Virginia and is surrounded by Fairfax County and the Falls Church to the southwest, the City of Alexandria to the southeast, and Washington, D.C. to the northeast directly across the Potomac River, which forms the county’s northern border. Other landforms also form county borders, particularly Minor’s Hill and Upton’s Hill on the west.
According to the U.S. Census Bureau, the county has a total area of 26.1 square miles (67.6 km2), of which 26.0 square miles (67.3 km2) is land and 0.1 square miles (0.3 km2) (0.4%) is water. It is the smallest county by area in Virginia and is the smallest self-governing county in the United States. About 4.6 square miles (11.9 km2) of the county is federal property. The county is roughly in the shape of a rectangle 4 miles (6.4 km) by 6 miles (9.7 km), with the small end slanting in a northwest-southeast direction. It has no incorporated areas. Its county seat is the census-designated place (CDP) of Arlington, which is coterminous with the boundaries of the county; however, the county courthouse and most government offices are located in the Courthouse neighborhood.
The Pentagon in Arlington is the headquarters of the United States Department of Defense. It was dedicated on January 15, 1943 and it is the world’s largest office building. Although it is located in Arlington, the United States Postal Service requires that „Washington, D.C.“ be used as the place name in mail addressed to the six ZIP codes assigned to The Pentagon.