This paper addresses the issue of how rules are conceptualized by Hodgson in Old Institutional Economics (OIE). The argument is put forward that the concept of rules can be constructively clarified. Rather than provide a general form of single rules within a rule system, we argue for a taxonomic range of single rule forms. This approach has the additional advantage of providing a more explicit account of how rules operate as part of a rule system. It also provides one way to address the fluidity of rules. Rules are understood to be more or less fluid (incomplete) and subject to a practical dynamism. This, we argue, can be differentiated from the idea of tendency based on the capacity of agents not to follow rules. A useful concept here is that of mezzo rules or recodifications that both define the rule and distance the agent from their own rule-following behaviour. In pursuing the argument we also highlight various methodological implications. First, conceptual development is a key aspect of the OIE, particularly when it is located within Dows structured pluralism. As such elaboration on rule forms enhances the consistency of OIE as methodology. Second, the exploration of a taxonomic range of rules and of forms of fluidity can provide useful resources in mapping out institutional processes in real research. Copyright © The JOIE Foundation 2010.