While treaties can be notoriously difficult to amend by formal means, they must nevertheless be adapted over time in order to remain useful. Herein lies the role of subsequent practice as a key tool for treaty change. Subsequent practice-a well-established means of treaty interpretation-sometimes diverges from the original treaty provision to such an extent that it can no longer be said to constitute an act of interpretation or application. Rather, it becomes, in effect, one of treaty modification.

The modification of treaties by subsequent practice extends to all fields of international...