Loudoun County () is located in the northern part of the Commonwealth of Virginia in the United States. In 2019, the population was estimated at 413,538, making it Virginia's third-most populous county. Loudoun County's seat is Leesburg. Loudoun County is part of the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area. As of 2018, Loudoun County had a median household income of $136,268. Since 2008, the county has been ranked first in the U.S. in median household income among jurisdictions with a population of 65,000 or more.

Middleburg is a town in Loudoun County, Virginia, United States, with a population of 673 as of the 2010 census. It is the southernmost town along Loudoun County's shared border with Fauquier County. Middleburg is known as the "Nation's Horse and Hunt Capital" for its foxhunting, steeplechases, and large estates. The Middleburg Historic District, comprising the 19th-century center of town, is listed on the National Register of Historic Places.

Montgomery County is the most populous county in the U.S. state of Maryland, located adjacent to Washington, D.C. As of the 2020 census, the county's population was 1,062,061, increasing by 9.3% from 2010. The county seat and largest municipality is Rockville, although the census-designated place of Germantown is the most populous city within the county. Montgomery County is included in the Washington–Arlington–Alexandria, DC–VA–MD–WV metropolitan statistical area, which in turn forms part of the Baltimore–Washington combined statistical area. Most of the county's residents live in unincorporated locales, of which the most urban are Silver Spring and Bethesda, although the incorporated cities of Rockville and Gaithersburg are also large population centers, as are many smaller but significant places.As one of the most affluent counties in the United States, Montgomery County also has the highest percentage (29.2%) of residents over 25 years of age who hold post-graduate degrees. The county has been ranked as one of the wealthiest in the United States. Like other inner-suburban Washington, D.C. counties, Montgomery County contains many major U.S. government offices, scientific research and learning centers, and business campuses, which provide a significant amount of revenue for the county.

Jefferson County is located in the Shenandoah Valley in the Eastern Panhandle of West Virginia, and is the easternmost county of the U.S. state of West Virginia. As of the 2010 census the population was 53,498. Its county seat is Charles Town. The county was founded in 1801, and today is part of the Washington metropolitan area.