The termination of U.S. treaties provides an especially rich example of how governmental practices can inform and even define the Constitution’s separation of powers. The authority to terminate treaties is not addressed specifically in the constitutional text and instead has been worked out over time through political branch practice. This practice, moreover, has developed largely without judicial review. Despite these features, Congress and the President — and the lawyers who advise them — have generally treated this issue as a matter of constitutional law, not merely political happenstance. Importantly, the example of treaty termination illustrates not only how historical practice can inform constitutional understandings, but also how these understandings can change. In the nineteenth century, the prevailing view was that the termination of treaties required either congressional or senatorial approval. That understanding changed in the twentieth century, and unilateral presidential termination became the norm. The contours of this shift likely apply to other issues of constitutional law relating to presidential authority: First there is a consensus, both among the governmental actors and in the scholarly community. Then deviations take place with a potentially limited scope. The Executive Branch proceeds to articulate broader theories of the deviations. Congress’s resistance is intermittent, depending on whether it objects to the deviations on policy grounds. Practice then builds up around low-stakes examples. Eventually a more controversial example arises and the President pushes forward successfully, thereby consolidating the changed understanding. This dynamic of accretion and consolidation of institutional practice is not adequately captured by theories of constitutional change that focus on judicial review or that emphasize particular moments of contestation or public deliberation.