Walter De Mulder et Anne Vanderheyden : L'histoire de contre et la sémantique prototypique Following the book on diachronic prototype semantics by Dirk Geeaerts, this paper tries to show that prototype semantics is, by its very nature, fit to describe semantic change. The authors first propose a semantic analysis of the Modern French preposition contre in terms of a "network", where the different meanings of the preposition (its spatial and adversative meanings, as well as the meanings of proportion and exchange) are organized around a prototypical sense ("movement of an entity towards another entity which functions as a limiting point") - an analysis that also includes a "schematic" or "basic" meaning. In the second part of the paper, they try to apply the same type of approach to analyze the use of the preposition in Old French and to explain the evolution of contre from Old French to Modern French. In this way, they can point out some phenomena which confirm the prototypical nature of the meaning-structure of contre: more than one meaning can be at the basis of one and the same extension of meaning, some meanings are more relevant than others in order to understand the semantic evolution of the preposition, and some meanings are more stable than others. Moreover, it is shown, following Geeraerts, that the prototypical conception of meaning can maintain the isomorphic principle ("one form, one meaning").