Ratified December 15, 1791
The Bill of Rights' closing statement of federalism: any power the Constitution doesn't give to the federal government, and doesn't forbid to the states, belongs to the states or to the people. This is the textual anchor for most modern "states' rights" arguments.
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.