These two cities, located about 30 miles apart, recently combined their metropolitan areas and now total 10,000 square miles, the fourth largest metropolitan area in the country, after NY, LA, and Chicago. Which two cities are these?

While these two major Mid-Atlantic cities have long maintained their own distinct identities, the U.S. government now officially recognizes them as a single entity for statistical purposes. This is because the suburbs and commuter corridors between the nation's capital and its "Charm City" neighbor have become so deeply intertwined. The official designation is the Baltimore-Washington Combined Statistical Area, or CSA. This classification is primarily based on economic and social ties, particularly the high volume of residents who live in one city's sphere of influence but commute to work in the other.

Despite their geographic proximity, the two cities have very different histories. Washington, D.C., was a planned city, purpose-built to be the seat of the federal government. Baltimore, on the other hand, grew organically as a major industrial port and manufacturing hub. Over the decades, the 30-mile stretch between them, once mostly rural, has filled in with sprawling suburban communities, federal agencies, and corporate offices.

This fusion has created a megalopolis of nearly 10 million people, making it one of the most populous and economically powerful regions in the country. The resulting 10,000-square-mile footprint is a testament to the growth that has blurred the lines between these two historic urban centers, solidifying the region's status as a true American megaregion.