Thirty-nine delegates signed the U.S. Constitution on September 17, 1787.
The signing took place at Independence Hall in Philadelphia during the final session of the Constitutional Convention. Delegates from twelve states were represented; Rhode Island had declined to send delegates. The convention had originally included more participants, but some left before the final ceremony.
Three delegates present at the convention refused to sign: George Mason, Edmund Randolph, and Elbridge Gerry. Several other delegates had already departed. The final group of 39 therefore represented neither every participant nor every state equally.
Benjamin Franklin, one of the signers, reportedly responded to a question about the convention’s result by observing that the delegates had created “a republic, if you can keep it.” The Constitution then moved to the states for ratification.