Congress will finally right a wrong by acknowledging the work of slaves who helped to build the U.S. Capitol building in Washington, D.C. in a ceremony planned for June 16.

During the ceremony, a plaque commemorating their work will be dedicated in the Rayburn Room. Among those scheduled to attend are Rep. John Lewis (D-Ga.), Sen. Blanche Lincoln (D-Ark.), House Minority Leader John Boehner (R-Ohio), Senate Majority Leader Harry Reid (D-Nev.), Senate Minority Leader Mitch McConnell (R-Ky.), and House Speaker Nancy Pelosi (D-Calif).

Its has taken more than two centuries for Congress to admit that those who built the monument to freedom weren’t free themselves, and the plaque is the first formal acknowledgement of their contribution.

However, the U.S. Capitol is far from the only historically significant American building built by slaves. Faneuil Hall in Boston, Redwood Library in Rhode Island, Independence Hall in Philadelphia as well as the homes of George Washington, Thomas Jefferson and James Madison in Virginia were all built with the help of, or primarily by, slaves.

The slaves who built the Capitol were rented from owners in the D.C. area, a practice which was commonplace according to a report issued by the Congress on the construction of the U.S. Capitol.

“George Washington, for instance, occasionally leased his slaves to neighbors who required their labor or skills for a short period of time,” the report states. “From time to time Washington found himself short of hands and was obliged to pay for additional help.”

According to the London Newspaper The Independent, between 1792 and 1800, 400 slaves were rented out from local plantations to build the White House and the Capitol. Each slave generated $5 a month, which was paid to the plantation owners of the leased slaves. Construction on the building lasted from 1793 until about 1826.