abstract. Just as private parties use contracts to facilitate joint projects and nation-states use treaties to organize joint undertakings, domestic governments use a breathtaking array of written instruments to coauthor legal rules and coordinate public programs. But we lack a vocabulary— literal and conceptual—to describe these agreements.

Our meager vocabulary does not reflect a meager practice. Intergovernmental agreements define the contours of public-benefits programs, cross-deputize police and immigration officers, facilitate the exchange of land and jurisdiction, manage vast flows of information, and more generally allow our levels of government to coauthor legal rules in a wide range of policy areas.

Nor is our impoverished vocabulary an indication of neglect from our judicial institutions. The Supreme Court, lower federal courts, and state courts routinely address disputes that arise from the distinctive multilateral nature of intergovernmental agreements. The central framework courts use to resolve such disputes is the private law of contract, yet they also adjust those contractual principles—often in an ad hoc way—to accommodate their public parties and public purposes.

By drawing these cases together across contexts, we can see doctrinal patterns, jurisprudential puzzles, and theoretical implications that stem from this dual character as both contract and public law. We can begin, for the first time, to build a treaty law for American federalism.

author. Sharswood Fellow, University of Pennsylvania Law School. I am grateful for thoughtful conversations with and comments from Payvand Ahdout, Ian Ayres, Shyam Balganesh, Niko Bowie, Jessica Bulman-Pozen, Ben Eidelson, Bill Eskridge, Jean Galbraith, Heather Gerken, Dave Hoffman, Becca Lee, Robert Post, Judith Resnik, Cristina Rodríguez, David Schleicher, Tess Wilkinson-Ryan, and especially Alex Hemmer. This Article also benefited from spirited workshops at Boston College, Columbia, Georgetown, Northwestern, the University of Chicago, the University of Pennsylvania, the University of Virginia, the University of Wisconsin-Madison, Washington University in St. Louis, and Yale. Thank you finally to the superb staff at the Yale Law Journal, particularly Lorraine Abdulahad and Soren Schmidt, for their editorial insight and diligence and to Andrew Lang at the University of Pennsylvania Biddle Law Library for his tenacity in locating sources.