In Arlington, Virginia, inside a small iron fence, a sandstone block carved "Virginia" and "1791" marks the edge of the District of Columbia. The District has not reached that spot since 1846.
Forty of these stones were set one mile apart around a perfect square in 1791 and 1792. Fourteen went into Virginia soil, and all fourteen survive on land Congress handed back to the state. Find Washington DC on a US map today and you are looking at what was left: a diamond with its southwest third cut away along the Potomac.
A capital drawn as a 10-mile square
The Residence Act of July 16, 1790 put the capital on the Potomac, in a district "not exceeding ten miles square." Washington's proclamation of March 30, 1791 fixed the starting point at Jones Point, the upper cape of Hunting Creek in Virginia, and ran the lines out from there, which pulled the port town of Alexandria inside.
Andrew Ellicott led the survey, with Benjamin Banneker as an assistant surveyor. The DC government credits Banneker with calculating the exact spot for the southern corner and setting the first stone at Jones Point on April 15, 1791. The Virginia stones were finished that year and the Maryland stones in 1792. When the DC government re-measured the line with GPS, the distance between two of the southeast stones came out at 5,280.824 feet - a mile, give or take 10 inches.
The square held 100 square miles: 69 from Maryland and 31 from Virginia.
The line in the fine print
Alexandria's residents applauded their inclusion, expecting the town to benefit economically.
The March 3, 1791 amendment that let the district take in Alexandria also said nothing in it could authorize "the erection of the public buildings otherwise than on the Maryland side" of the Potomac. The Capitol and the President's House were built on the Maryland side.
Living in a capital without a vote
Congress took direct control of the district with the Organic Act of 1801. Starting with the 1802 election, Alexandrians could no longer vote for members of Congress, and more of their local government was appointed rather than elected.
Alexandria could not compete with Georgetown as a port. It counted 8,218 residents in 1820 and 8,459 in 1840, a gain of 241 people in 20 years. The town dug the Alexandria Canal, opened in 1843, to connect with the Chesapeake and Ohio Canal at Georgetown and win back its trade.
The slave trade in the capital's back yard
Beginning in the 1820s, Alexandria also became a major port of the domestic slave trade. From 1828 to 1837, Franklin and Armfield ran a slave jail at 1315 Duke Street and shipped more than 3,750 enslaved people to cotton and sugar plantations in Louisiana, on two ships of their own.
That put the trade inside a federal district, where Congress could end it. Abolitionists hoped it would. Pro-slavery Virginians wanted Alexandria back for two more pro-slavery seats in the state assembly. Majorities from both free and slave states voted for retrocession, and historians cited in the WETA Boundary Stones account question how much slavery decided the outcome.
The vote of 1846
The House passed retrocession 96 to 65 and the Senate 32 to 14. President James K. Polk signed it on July 9, 1846.
The act still needed local approval. On September 1 and 2, qualified freeholders came to the Alexandria courthouse and voted aloud. The count, announced in Polk's proclamation of September 7, was 763 for and 222 against.
The town voted 734 to 116 to leave. The rural county around it - today's Arlington - voted 106 to 29 to stay in the District, and was outvoted. Virginia accepted the land on March 13, 1847.
After the line moved
Four years later, as part of the Compromise of 1850, Congress banned the slave trade in the District of Columbia. Slavery itself stayed legal there until April 16, 1862. Alexandria was now in Virginia, where the ban did not apply. The sign reading "Price, Birch & Co. Dealers in Slaves" was still on the Duke Street building when photographers recorded it in August 1863.
After retrocession, Alexandria's African American community saw schools close.
The move was challenged once in court. In Phillips v. Payne in 1875, a taxpayer argued that handing part of the District back was unconstitutional. The Supreme Court ruled that Virginia was in actual possession of the land, that Congress had recognized the transfer, and that he could not challenge it.
Where the square still shows
Thirty-six of the 40 stones remain in or near their original spots. The District covers 68 square miles, 61 of them land, and three of its sides are still the straight lines Ellicott's crew surveyed. The fourth follows the Virginia shore of the Potomac.
At the scale of a full US map, DC is a dot between Maryland and Virginia, and the missing corner disappears. To show it, draw the original square on our free US map with states, which you can download as an SVG. The story of US county lines covers how the rest of the country's local boundaries were drawn.