The US Constitution contains no provision for its own replacement (and because of the presence of entrenched clauses it cannot be revoked through an amendment) so any constituent assembly would necessarily be extra-constitutional. Article V of the Constitution does permit Congress to appoint a national constitutional convention to propose amendments but it has never done so. While Congress has the option to submit both its own proposals for amendments and those of a national convention to state conventions rather than the state legislatures for ratification; this process has been used only once (for the Twenty-first Amendment). Neither of these would necessarily be constituent assemblies, as there is no provision requiring that such bodies be elected rather than appointed by the existing legislature.