Elinor Ostrom's Institutional Analysis and Development (IAD) framework has been described as ‘one of the most developed and sophisticated attempts to use institutional and stakeholder assessment in order to link theory and practice, analysis and policy’. But not all elements in the framework are sufficiently well developed. This paper focuses on one such element: the ‘rules-in-use’ (a.k.a. ‘rules’ or ‘working rules’). Specifically, it begins a long-overdue conversation about relations between formal legal rules and ‘working rules’ by offering a tentative and very simple typology of relations. Type 1: Some formal legal rules equal or approximate the working rules; Type 2: Some legal rules plus (or emended by) widely held social norms equal or approximate the working rules; and Type 3: Some legal rules bear no evident relation to the working rules. Several examples, including some previously used by Ostrom, are provided to illustrate each of the three types, which can be conceived of as nodes or ranges along a continuum. The paper concludes with a call for empirical research, especially case studies and meta-analyses, to determine the relevant scope of each of these types of relations, and to provide data for furthering our understanding of how different types of rules, from various sources, function (or not) as institutions.