The SC (SINIX* Consultant) system is a natural language help facility for the SZNIX operating system. In order to offer individual help to a user, SC uses assumptions about the knowledge of the user. The user modeling module SC-UM provides a priori knowledge in the form of prototypical user models gained from a study of the typical knowledge of SINIX users with different levels of sophistication. In addition, an individual user model is constructed during the course of a session. Each update of the individual user model causes a new assignment of the user to a prototypical user model.