The quality of vowels in French depends to a large extent on the kind of syllables they are in. Tense vowels are often in open syllables and lax vowels in closed ones. This generalisation, which has been called loi de position in the literature, is often overridden by special vowel-consonant co-occurrence restrictions obscuring this law. The article shows first that the admission of semisyllables in the phonology of French explains a large number of counterexamples. Many final closing consonants on the phonetic representation can be understood as onsets of following rime-less syllables, opening in this way the last full syllable. Arguments coming from phonotactic regularities support this analysis. The second insight of the article is that Optimality Theory is a perfect framework to account for the intricate data bearing on the relationship between vowels and syllable structure. The loi de position is an effect dubbed Emergence of the Unmarked, instantiated only in case no higher-ranking constraint renders it inactive.