x-bar theory approaches categorical features in this way: when a head x selects its complement to project to x', the xp that it projects to then is a combination of the head x and all of its categorical features, those being either nominal, verbal, adjectival or prepositional features. it has also been argued that adpositions (cover term for prepositions and postpositions) are not part of the [+/-n] [+/-v] system as shown above. this is because they resist being part of a single class category, like nouns, verbs and adjectives do. this argument also posits that some appositions may behave as part of this type of categorization, but not all of them do.