From Latin to French, an article took shape out of a preposition with a concrete spatial meaning. This paper highlights two crucial stages in the grammaticalization process leading from the preposition de to the so-called 'partitive' article: the Old French stage, which is the last prepositional stage of de, and the Middle French stage, where de is entering into the paradigm of articles. Two central issues are dealt with: (i) Why did French develop an article for indefinite non-singular reference, whereas most other languages endowed with an article system leave this type of reference unmarked? (ii) How can we, at the pre-article stage, account for the use of the preposition de in syntactic functions that are in principle incompatible with the use of a preposition? The study also reflects upon the theoretical postulate of gradualness of the grammaticalization process and raises the question of a possible correlation between a shift in morphosyntactic category and a discontinuity in evolution.