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Subsequent agreements and practices are important mechanisms by which treaty parties can assert their continued interpretive authority over treaties without having to amend those treaties. States are masters of their treaties not only in the sense that they can enter into and exit from such treaties, but also because the Vienna Convention on the Law of Treaties requires that treaties be interpreted in light of the treaty parties’ subsequent agreements and practices that are relevant to interpretation. This provision recognizes the significant and ongoing role that treaty parties are permitted to play in the interpretation and application of their own treaties over time. However, the claim that treaty parties have interpretive authority over their treaties exists in tension with the claim to interpretive authority by international courts and tribunals. In theory, states and states alone make international law while decisions of international courts and tribunals are merely subsidiary means of determining international law rather than sources of international law per se. In practice, when states delegate power to international courts and tribunals to resolve disputes under treaties and/or to interpret and apply those treaties, they impliedly delegate some law-making functions to those judicial bodies. Those judicial decisions are then routinely cited as evidence of what the law is, even when these decisions clearly develop rather than merely apply the law.
Roberts, Anthea, Subsequent Agreements and Practice: The Battle Over Interpretive Power (August 23, 2013). Treaties and Subsequent Practice, Georg Nolte ed., 2013, Available at SSRN: https://ssrn.com/abstract=2315134