The aim of this paper is to improve the description of root (or non-epistemic) possibility meanings. In previous accounts, the defining criteria are not applied systematically; there is a tendency towards definition by exemplification (especially when it comes to meanings that are 'not permission' and 'not ability') and certain categories (permission, for instance) tend to be defined in a circular way. We will argue that there are three criteria which are necessary and sufficient to distinguish five subclasses of root possibility meaning. The three criteria are: (a) the scope of the modal meaning, (b) the source of the modality and (c) the notion of potential barrier; the five meanings are: (a) ability, (b) opportunity, (c) permission, (d) general situation possibility (GSP) and (e) situation permissibility. The paper offers an in-depth analysis of the three defining criteria and the root possibility meanings that their systematic application gives rise to. This approach clearly brings out the similarities and the dissimilarities between the different subcategories of root possibility meaning in English and in this way it results in a more explicit taxonomy.