Nine states had to ratify the United States Constitution for it to take effect.
Article Seven set the Constitution’s ratification threshold at nine of the thirteen states. This was a major departure from the Articles of Confederation, which required unanimous approval for important changes. The new rule allowed the proposed federal government to begin even if some states initially withheld approval.
New Hampshire became the ninth state to ratify the Constitution on June 21, 1788. That vote placed the Constitution into effect, although North Carolina and Rhode Island had not yet ratified it. Rhode Island ultimately became the last of the original states to approve the document, doing so in 1790.
The nine-state threshold is sometimes confused with the number of states that had ratified by the time the new government began operating. Eleven states had ratified by September 1788, allowing arrangements for the first Congress and president to proceed.