Local governments regularly use formal interlocal cooperation to deliver public services, including in complex infrastructure systems. Theories of social networks and institutional collective action have motivated an expansive body of scholarship on the institutional determinants (rules, norms, strategies) of such cooperation. Building off recent advances in the conceptualization and use of institutional grammar, we identify an additional institutional determinant: constitutive institutions in the form of tacit understandings about the fundamental parameters of the infrastructure system, including their orientation to cooperation, systemicity, and a common performance paradigm. More plainly, we propose that interlocal cooperation will be more likely where decision-makers and stakeholders in the governance choice action arena have similar institutional imaginations of the system. Because these imaginations are unlikely to leave written artifacts, we relied instead on deductive coding of interviews with system elites. We use this paper to introduce a conceptual framework and test the viability of institutional grammar as a technique for exploring unwritten constitutive institutions. We focus our work on drinking water systems in the highly polycentric Chicagoland region, where local governments enjoy substantial discretion in governance choice.