The 1958 debate between Lon Fuller and H.L.A. Hart in the pages of the Harvard Law Review is one of the landmarks of modern jurisprudence. Much of the debate was about the relative merits of Hart’s version of legal positivism and Fuller’s brand of natural law theory, but the debate also contained the memorable controversy over the fictional rule prohibiting vehicles from the park. Hart used the example to maintain that rules have a core of clear applications surrounded by a penumbra of uncertainty, but Fuller offered a counterexample to insist that the language of a rule, by itself, could never determine any legal outcome. At one level, therefore, the debate was about the relative importance of language and purpose in applying a general rule to a particular issue. At a deeper level, however, the debate was about the formality of law and about the possibility of varying commitments to formality in different legal systems. By examining this debate, and by largely removing it from the surrounding controversy over positivism and natural law, we can gain valuable insights about legal rules, legal interpretation, and the nature of legal language.