Article Seven of the United States Constitution
The Congress of the Confederation decided that the United States would start using the new Constitution on March 4, 1789.  Before that day, Virginia and New York ratified the Constitution.  However, when the first United States Congress gathered on March 4 to form the new government, North Carolina and Rhode Island still had not approved the Constitution.  After the new Congress added the Bill of Rights to the Constitution, though, North Carolina and Rhode Island ratified the Constitution.