Who had to ratify (approve) the Constitution before it could take effect?
9 of the 13 original states apparently did the trick.
Here's a fun trivia question: How many states were in the Union when George Washington first took office? Eleven. While the earliest flags had 13 stars for the original colonies, the truth is, there were two holdouts as of when the first president took office: North Carolina (November 21, 1789) and Rhode Island (May 29, 1790). The latest signatory of the prior eleven, New York, ratified on July 26, 1788, early enough to hold the first (constitutional) election in 1788.
Actually, the Constitution was ratified on the same day that the ninth state approved it: Jun 21, 1788, the day New Hampshire signed on. Virginia was next, then New York.