The word “structure” is now too often used as a password. But in linguistics it appears to have been adopted independently, before the current vogue. Here it has retained its most narrow connotation. The study of linguistic structures means the study of the construction of certain linguistic fragments: the attempt to uncover, according to their linguistic function, the real units that make up these fragments and the rules for using these units in constructing the fragments. This method of analysis, functional and structural, forty years ago transformed the description of language, whether it was a question of phonemics, of morphology, or even of syntax. But for a long time the structuralists themselves insisted upon the resistances that semantics raise—and its most apparent formal manifestation, the lexicon—to any attempt at analysis of this sort.