Article VII of the U.S. Constitution required nine states to ratify it for the Constitution to take effect.
The nine-state threshold was a compromise between requiring unanimous approval and allowing a small minority of states to establish the new system. The Constitution therefore did not require all thirteen states to ratify it before becoming operative among the participating states.
New Hampshire became the ninth state to ratify on June 21, 1788. Its decision met Article VII’s threshold, although Virginia and New York had not yet ratified. North Carolina and Rhode Island joined later, with Rhode Island becoming the final original state to ratify in 1790.
The ratification rule is often confused with the total number of original states. Thirteen states participated in the ratification process, but nine was the constitutional minimum needed to activate the new framework.