This dissertation investigates which syntactic features are concerned with the linguistic expression of polarity by discussing their nature, their combinations, and their hierarchy. A featural machine summarizes the salient properties within a three-part list of some computational conditions involved in the projection of grammatical sentences in natural languages. According to various theoretical and empirical evidences, we argue against unitary concepts of "negativity" and "positivity", which have rather to be split up in several polarity-related features. Then we draw up a polar grammar of French by taking an empirical inventory of about 750 polarity items in the guise of a database; the featural analysis of the contexts where polarity phenomena arise provides the basis to set up a range of criteria allowing to correctly settle the 16'000 switches of the chosen polarity items, so that the database could be processable by the featural machine.