The model proposed here is used to describe the vocabulary of a corpus. It is divided into two groups : general vocabulary which is used whatever the circumstances and several local (or 'specialised') vocabularies, each of which is used in only one part of the corpus, General words may appear everywhere in the text and their increase with corpus size can be estimated with Muller's formula. In this model, a partition parameter measures the relative importance of both types of vocabularies: so the value of this parameter gives an estimation of the lexical 'specialisation' in the text. This model has been applied to Racine's plays and TV debates (Giscard vs Mitterrand, Chirac vs Fabius). The partition model can also be used to measure the increase of vocabulary with corpus length, to locate stylistic changes or to compare several texts from the point of view of their lexical richness.