The object of analysis in the paper is semantic extension of a lexical unit. In order to approach it, the author chooses one of the cognitive linguistics theories – Cognitive Grammar (Langacker 1987; 1990; 2000a; 2000b; 2008, etc.). Two of the issues of semantic extension are emphasised. First, it is the grounding of semantic extension in the encyclopaedic knowledge shared by the interlocutors and second, the emergence of the schema implied by the relation of extension. The paper begins with an outline of the postulates of Cognitive Grammar, which are subsequently applied to an analysis of the French lexical unit corps [body], whose extended senses are found in the domain of the structure of musical instruments. In the conclusion the author discusses the dimensions of complexity of the process of semantic extension, one of which is a chain of relations based on metonymy and metaphor.