Summary: User interfaces often only support one way to do a task when the physical interface or the requirements of the task would permit other ways. In contrast, a user interface that supports multiple approaches is permissive. This paper argues that permissive user interfaces are easier to use--and even when they are not applicable for particular applications, considering permissiveness is a productive design heuristic. Many user interfaces are difficult to use yet very easily demonstrated or explained by experts--with the results that users become frustrated because hindsight makes usability problems look like the user's own fault. The lack of permissiveness in such user interfaces explains this paradox.