 A promising candidate for the role of secondary rule is the Vienna Convention on the Law of Treaties, which is generally understood to reflect customary international law. As a treaty about treaties, the Convention has come to perform a systemic function by regulating the adoption, entry into force, amendment and interpretation of treaties. By providing rules for treaty-making, the Vienna Convention operates for treaties much like the Uniform Commercial Code in the US or the Code Civil in France do for contracts. It is, in other words, a set of primary and secondary rules that enables certain promises to have legal effect.