Arlington County is a county in the Commonwealth of Virginia, often referred to simply as Arlington or Arlington, 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, under the name Alexandria County. The county is coterminous with the U.S. Census Bureau's census-designated place of Arlington. Arlington 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 the Virginia Law. In 2019, the county's population was estimated at 236,842, making Arlington the sixth-largest county in Virginia by population; if it were incorporated as a city, Arlington would be the fourth most populous city in the state. With a land area of 26 square miles (67 km2), Arlington is the geographically smallest self-governing county in the U.S., and by reason of state law regarding population density, it has no incorporated towns within its borders. It is the 5th highest-income county in the U.S. by median family income, and is the 11th most densely populated county in the United States. Arlington is home to the Pentagon, Reagan National Airport, and Arlington National Cemetery. In academia, the county contains Marymount University, George Mason University's Antonin Scalia Law School, the administrative offices buildings and graduate programs for the Schar School of Policy and Government and the Jimmy and Rosalynn Carter School for Peace and Conflict Resolution, as well as satellite campuses of the University of Virginia and Virginia Tech. It will soon house Amazon HQ2, the co-headquarters of Amazon.

Alexandria is an independent city in the Commonwealth of Virginia in the United States. As of 2010, the population was 139,988, and in 2020, the population was estimated to be at 159,200. The city's estimated population has grown by 1% percent annually since 2010 on average. Located along the western bank of the Potomac River, Alexandria is approximately 7 miles (11 km) south of downtown Washington, D.C. Like the rest of Northern Virginia, as well as Central Maryland, modern Alexandria has been influenced by its proximity to the U.S. capital. It is largely populated by professionals working in the federal civil service, in the U.S. military, or for one of the many private companies which contract to provide services to the federal government. One of Alexandria's largest employers is the U.S. Department of Defense. Another is the Institute for Defense Analyses. In 2005, the United States Patent and Trademark Office moved to Alexandria, and in 2017, so did the headquarters of the National Science Foundation. The historic center of Alexandria is known as Old Town. With its concentration of boutiques, restaurants, antique shops and theaters, it is a major draw for all who live in Alexandria as well for visitors. Like Old Town, many Alexandria neighborhoods are compact and walkable. It is the 7th largest and highest-income independent city in Virginia. A large portion of adjacent Fairfax County, mostly south but also west of the city, has Alexandria mailing addresses. However, this area is under the jurisdiction of Fairfax County's government and separate from the independent city. The city is therefore sometimes referred to as the "City of Alexandria" to avoid confusion (see the "Neighborhoods" paragraph below). Additionally, neighboring Arlington County was formerly named "Alexandria County" before it was renamed by the Virginia General Assembly in 1920 to reduce confusion with the city.

National Harbor is situated on the banks of the historic Potomac River, just minutes from the nation’s capital. Featuring 350 acres of waterfront resort space, National Harbor is a destination unlike any other.

Prince George's County (often shortened to PG County) is a county located in the U.S. state of Maryland, bordering the eastern portion of Washington, D.C. As of the 2010 U.S. Census, the population was 863,420, making it the second-most populous county in Maryland, behind Montgomery County. Its county seat is Upper Marlboro. It is the largest and the most affluent African American-majority county in the United States, with five of its communities identified in a 2015 top ten list.Prince George's County is included in the Washington metropolitan area. The county also hosts many federal governmental facilities, such as Joint Base Andrews and the United States Census Bureau headquarters.

Upper Marlboro, officially the Town of Upper Marlboro, is the seat of Prince George's County, Maryland, United States. The population within the town limits was 631 at the 2010 U.S. Census, although Greater Upper Marlboro is many times larger.