We analyse the relationship between applicability and effectiveness of legal norms from a philosophical perspective. In particular, we distinguish between two concepts of applicability. The external applicability of norms refers to institutional duties; a norm N is externally applicable if and only if a judge is legally obliged to apply N to some case c. Internal applicability refers instead to the sphere of validity of legal norms. A norm N is internally applicable to actions regulated by its sphere of validity. (...) We also explore the consequences of a thesis which maintains that applicability restricts the concept of effectiveness, so that only applicable norms can be considered effective. Our analysis illustrates that a proper reconstruction of the concept of applicability is of great importance not only for understanding the concept of effectiveness but also for providing insight into the nature of law. (shrink)