The ‘Gänseliesel’ (Goose Girlis), a historical fountain erected in 1901, represents the most well-known landmark of the city of Goettingen.

Subsequent Practice and Established Practice of International Organizations: Two Sides of the Same Coin?

Christopher Peters



The present Article considers and compares the subsequent practice of the parties according to Art. 31 (3) (b) VCLT and established practice amounting to rules of an international organization (Art. 5 VCLT). The significance of these concepts lies in their potential to contribute to the adaptation of constituent instruments of international organizations to changing factual and normative circumstances. Established practice can serve as a hinge between the general law of treaties and the law of international organizations. The paper argues that both concepts are not two sides of the same coin, but that they have to be distinguished. Whereas subsequent practice primarily serves in interpretation, established practice amounting to a rule of the organization is quasi-customary law specific to the respective organization. It can even influence the preconditions for and significance of subsequent practice in the application of constituent instruments. Thus, the requirements for the agreement of the parties in accordance with Art. 31 (3) (b) VCLT can be relaxed and tacit consent can be recognized more easily. In some cases even organ practice which is independent from (all) Member States can create subsequent practice. However, these informal mechanisms of change raise problems of legitimacy.


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