It has been perceived that languages tend to be organized around grammatical categories such as noun and verb, nominative and accusative, or present and past, though not exclusively so. The grammar of a language is organized around such fundamental categories, though many languages express the relationships between words and syntax in other discrete ways (cf. some Bantu languages for noun/verb relations, ergative-absolutive systems for case relations, several Native American languages for tense/aspect relations).
In addition to making substantial use of discrete categories, language has the important property that it organizes elements into recursive structures; this allows, for example, a noun phrase to contain another noun phrase (as in "the chimpanzee's lips") or a clause to contain a clause (as in "I think that it's raining"). Though recursion in grammar was implicitly recognized much earlier (for example by Jespersen), the importance of this aspect of language became more popular after the 1957 publication of Noam Chomsky's book Syntactic Structures,^[14] which presented a formal grammar of a fragment of English. Prior to this, the most detailed descriptions of linguistic systems were of phonological or morphological systems.
Chomsky used a context-free grammar augmented with transformations. Since then, following the trend of Chomskyan linguistics, context-free grammars have been written for substantial fragments of various languages (for example, GPSG, for English). It has been demonstrated, however, that human languages (the most notable ones being Dutch and Swiss German) include cross-serial dependencies, which cannot be handled adequately by context-free grammars.^[15]