This paper analyzes the discursive activity of an upper secondary school teacher on the topic of the derivative. Specifically, we focus on the discursive activity when introducing the three essential concepts related to this topic: the average rate of change, the derivative at a point, and the derivative function. The theoretical framework used is the theory of commognition (Sfard, 2008). Using this framework, we identify different types of visual mediators (symbols, graphs, gestures) and routines (mainly construction and substantiation ones, with various subtypes). The analysis of these visual mediators and routines allows us to examine some resources students will have in their transition to university discourse. We also draw some other conclusions and present implications for teaching in this stage.