We show in this paper that all the uses of QU- words, as indefinite pronouns as well as connective words, can be explained from a lexical and a predicative constraint; the first constraint, of semantic nature, excludes any determinate use inisolation, the form being interpreted as a free variable; the second constraint imposes, even in polarity contexts, a double prédication resulting in obligatory morphological extensions, even in the pronominal uses, these extensions being more or less grammaticalised.