Howard County is located in the U.S. state of Maryland. As of the 2010 census, the population was 287,085. As of the 2020 census its population rose to 328,200. Its county seat is Ellicott City.Howard County is included in the Baltimore-Columbia-Towson, MD Metropolitan Statistical Area, which is also included in the Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area. Recent south county development has led to some realignment towards the Washington, D.C. media and employment markets. The county is home to Columbia, a major planned community of approximately 100,000 founded by developer James Rouse in 1967. Howard County is frequently cited for its affluence, quality of life, and excellent schools. Its estimated 2016 median household income of $120,194 raised it to the second-highest median household income of any U.S. county. Many of the most affluent communities in the area, such as Clarksville, Dayton, Glenelg, Glenwood, and West Friendship, are located along the Route 32 corridor in Howard County. The main population center of Columbia/Ellicott City was named second among Money magazine's 2010 survey of "America's Best Places to Live."In 2010, the center of population of Maryland was located in the Howard County town of Jessup.

Leesburg is the county seat of Loudoun County, Virginia. It was built in 1740 and is named for the Lee family, early leaders of the town and ancestors of Robert E. Lee. In the War of 1812, it was a refuge for important federal documents evacuated from Washington, DC, and in the Civil War, it changed hands several times. The town is situated at the base of Catoctin Mountain and adjacent to the Potomac River, 33 miles (53 km) northwest of Washington, D.C., for which it has largely become a commuter suburb. Its population was 42,616 as of the 2010 census and an estimated 53,727 in 2019. It is Virginia's largest incorporated town within a county (rather than being an independent city).

Frederick is a city in, and the county seat, of Frederick County, Maryland. It is part of the Baltimore–Washington Metropolitan Area. Frederick has long been an important crossroads, located at the intersection of a major north–south Native American trail and east–west routes to the Chesapeake Bay, both at Baltimore and what became Washington, D.C. and across the Appalachian mountains to the Ohio River watershed. It is a part of the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area, which is part of a greater Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area. The city's population was 65,239 people at the 2010 United States Census, making it the second-largest incorporated city in Maryland, behind Baltimore. Frederick is home to Frederick Municipal Airport (IATA: FDK), which accommodates general aviation, and to the county's largest employer U.S. Army's Fort Detrick bioscience/communications research installation.

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.