SVO, which is a much more common order among prepositional languages. Universals may also be or . In a bidirectional universal two features each imply the existence of each other. For example, languages with postpositions usually have SOV order, and likewise SOV languages usually have postpositions. The implication works both ways, and thus the universal is bidirectional. By contrast, in a unidirectional universal the implication works only one way. Languages that place relative clauses before the noun they modify again usually have SOV order, so pre-nominal relative clauses imply SOV. On the other hand, SOV languages worldwide show little preference for pre-nominal relative clauses, and thus SOV implies little about the order of relative clauses. As the implication works only one way, the proposed universal is a unidirectional one. Linguistic universals in syntax are sometimes held up as evidence for universal grammar (although epistemological arguments are more common). Other explanations for linguistic universals have been proposed, for example, that linguistic universals tend to be properties of language that aid communication. If a language were to lack one of these properties, it has been argued, it would probably soon evolve into a language having that property. Michael Halliday has argued for a distinction between and categories in resolving the matter of the existence of linguistic universals, a distinction he takes from J.R. Firth and Louis Hjelmslev. He argues that "theoretical categories, and their inter-relations construe an abstract model of language...; they are interlocking and mutually defining". Descriptive categories, by contrast, are those set up to describe particular languages. He argues that "When people ask about 'universals', they usually mean descriptive categories that are assumed to be found in all languages. The problem is there is no mechanism for deciding how much alike descriptive categories from different languages have to be before they are said to be 'the same thing'" Universal grammar Noam Chomsky's work related to the innateness hypothesis as it pertains to our ability to rapidly learn any language without formal instruction and with limited input, or what he refers to as a poverty of the stimulus, is what began research into linguistic universals. This led to his proposal for a shared underlying grammar structure for all languages, a concept he called universal grammar (UG), which he claimed must exist somewhere in the human brain prior to language acquisition. Chomsky defines UG as "the system of principles, conditions, and rules