The Rei(g)n of Rule is a study of rules and their role in language. Rules have dominated the philosophical arena as a fundamental philosophical concept. Little progress, however, has been made in reaching an accepted definition of rules. This fact is not coincidental. The concept of rule is expected to perform various, at times conflicting, tasks. Analyzing key debates and rule related discussions in the philosophy of language I show that typically rules are perceived and defined either as norms or as conventions. As norms, rules perform the evaluative task of distinguishing between correct and incorrect actions. As conventions, rules describe how certain actions are actually undertaken. As normative and conventional requirements do not necessarily coincide, the concept of rule cannot simultaneously accommodate both. The impossibility to consistently define ‘rule’ has gone unnoticed by philosophers, and it is in this sense that ‘rule’ has also blocked philosophical attempts to explain language in terms of rules.