(such as Al-Sayyid Bedouin Sign Language) developing in communities of congenitally deaf people, independently of spoken language. The properties of these sign languages conform generally to many of the properties of spoken languages. Other known and suspected sign language isolates include Kata Kolok, Nicaraguan Sign Language, and Providence Island Sign Language.StructuresFerdinand de SaussureIt 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, importantly, 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 recursivestructures; 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 (most notably Dutch and Swiss German) include cross-serial dependencies, which cannot be handled adequately by context-free grammars.Selected sub-fieldsHistorical linguisticsMain article: Historical linguisticsHistorical linguistics studies the history and evolution of languages through the comparative method. Often the aim of historical linguistics is to classify languages in language familiesdescending from a common ancestor. This involves comparison of elements in different languages to detect possible cognates in order to be able to reconstruct how different languages have changed over time. This also involves the study of etymology, the study of the history of single words. Historical linguistics is also called "diachronic linguistics" and is opposed to "synchronic linguistics" that study languages in a given moment in time without regarding its previous stages. In universities in the United States, the historic perspective is often out of fashion. Historical linguistics was among the first linguistic disciplines to emerge and was the most widely practiced form of linguistics in the late 19th century. The shift in focus to a synchronic perspective started with Saussure and became predominant in western linguistics with Noam Chomsky's emphasis on the study of the synchronic and universal aspects of language.SemioticsMain article: SemioticsSemiotics is the study of sign processes (semiosis), or signification and communication, signs and symbols, both individually and grouped into sign systems, including the study of how meaning is constructed and understood. Semioticians often do not restrict themselves to linguistic communication when studying the use of signs but extend