A General Rule Markup Language has several purposes. It may serve as a lingua franca for different rule systems and rule components in application software to exchange rules between each other. It may be used to express derivation rules for enriching XML-based taxonomies (also called `web ontologies') by adding definitions of derived concepts. It may be used to publish the reactive behavior of a system in the form of reaction rules. And it may be used to provide a complete XML-based specification of a software agent. Further uses may arise in novel web applications. In this paper, I consider the problem of how to design a General Rule Markup Language that can be used for these, and for future emerging purposes.