Here’s an interesting bit of American urban and legal history. In 1791 and 1792, a team led by Andrew Ellicott (including the famous Benjamin Banneker) surveyed the boundaries of the original District of Columbia.
In its first iteration, the District was shaped like a diamond, each side ten miles in length. It comprised the limits of the present city of Washington, whose lands were donated by Maryland, and a somewhat smaller area on the Virginia side of the Potomac.
In 1849, Virginia recovered its portion of the federal district, leaving only Maryland’s former portion as the federal district. So, today, a map of the City of Washington, D.C., looks like this:
Meanwhile, most of the land that Virginia reclaimed became today’s Arlington County, which retains the corresponding geometry of the original diamond for much of its boundary. The remainder is now in the independent city of Alexandria.
Today, the boundary stones set out by Ellicott, Banneker, et al., still, for the most part, exist — including those defining Virginia’s former portion of the District, which, in an alternate history, might also have remained legally a part of the nation’s capital.
You can learn more, and see all of the remaining D.C. markers, by visiting boundarystones.org.