Arlington County is a county in the Commonwealth of Virginia. The county is situated in Northern Virginia on the southwestern bank of the Potomac River directly across from the District of Columbia, of which it was once a part. The county is coextensive with the U.S. Census Bureau's census-designated place of Arlington. Arlington County is considered to be the second-largest "principal city" of the Washington metropolitan area, although Arlington County does not have the legal designation of independent city or incorporated town under Virginia state law.