Article Seven of the United States Constitution required nine states to ratify it for adoption. That threshold was two-thirds of the thirteen states existing during the ratification process.
The requirement was significant because the Articles of Confederation had required unanimous approval for major changes. Article Seven instead established a new constitutional government once nine state conventions agreed, even though the remaining states had not yet ratified.
New Hampshire became the ninth state to ratify on June 21, 1788. Its decision provided the constitutional threshold, although the first government under the Constitution began operating later, after elections and the inauguration of George Washington in 1789.
The final two original states eventually joined the constitutional system. North Carolina ratified in November 1789, while Rhode Island became the thirteenth and last original state to ratify in May 1790. Thus, nine was the legal threshold, not the final total.