Legislation that seems unreasonable to courts is less likely to be followed. Building on this premise, we propose a model and obtain two main results. First, the enactment of legislation prohibiting something raises the probability that courts will allow related things not expressly forbidden. In particular, the imposition of an interest rate ceiling can make it more likely that courts will validate contracts with interest rates below the legislated cap. Second, legal uncertainty is greater with legislation that commands little deference from courts than with legislation that commands none. We discuss examples of e§ects of legislated prohibitions (and, in particular, usury laws) that are consistent with the model.