Between vocabulary and grammar : verbal periphrasis of French

Verbal circumlocutions hesitate between lexicon and grammar. We analyze those construct on the verbs "aller" "venir" and "être en train de" under the angle diachronic, from its origins to the Present day. So, diverse themes will be approach : question of grammaticalization, choice between auxiliary and semi-auxiliary, semantic weakening, question of auxiliarization...The prespositional forms are study, which a particular concern granted in role of presposition in quest.