Languages from at least five genetically unrelated families are spoken in the Caucasus, but there are only three endemic linguistic families belonging to the region: Kartvelian, West Caucasian, and Northeast Caucasian. These families are rather heterogeneous in terms of the number of languages and the distribution of the speakers across them. The Caucasus represents a situation where languages with millions of speakers have coexisted with one-village languages for hundreds of years, and where multilingualism has always been the norm. The richness of Caucasian languages on every linguistic stratum is dazzling: here we find some of the largest consonant inventories, inflectional systems where the mere number of word forms strains credibility (one of the Caucasian languages, Archi, is claimed to have over a million and a half word forms), and challenging syntactic structures. The typological interest of the Caucasian languages and the challenges they present to linguistic theory lie in different areas. Thus, for Kartvelian languages, the number of factors at play in the verbal system make the task of the production of a correct verbal form far from trivial. West Caucasian languages represent an instance of polysynthetic polypersonal verb inflection, which is unusual not only for Caucasus but for Eurasia in general. East Caucasian languages have large systems of non-finite forms which, unusually, retain the ability to realize agreement in gender and number while their non-finite nature is determined by the inability to head an independent clause and to express certain morpho-syntactic categories such as illocutionary force and evidentiality. Finally, all Caucasian languages are ergative to some extent.
The five languages of the Northwest Caucasian family (also known as West Caucasian or Abkhaz-Adyghe), namely West Circassian, Kabardian, Ubykh, Abkhaz, and Abaza, are remarkable for their high degree of polysynthesis. This manifests itself in complex words that bear a lot of information on arguments and the characteristics of a situation, and which presumably can be constructed in the course of speech. Content words usually consist of several morphological zones within which certain permutations of morphemes are possible. Both prefixes and suffixes occur, with some morphemes being capable of appearing either as a prefix or a suffix, depending on the form. The predicate shows ergative-based cross-reference of core arguments and indirect objects introduced by applicatives, extensive use of the causative (including double causativization), highly developed means of expressing locational semantics within the predicate, and intricate tense-modality-aspect and polarity systems. Although classical noun-to-verb incorporation does not occur, there are constructions akin to incorporation, especially in the nominal domain. Nouns constitute a subclass of a broad class of predicates (both morphologically and syntactically) and hence may take the basic predicate morphology. At the same time, they form word-like nominal complexes with their attributes, and show specific head-marking possessive morphology (in West Circassian even distinguishing alienable and inalienable possession). Definiteness/specificity is regularly expressed either by articles (in Abkhaz, Abaza and Ubykh) or by the presence/absence of core case marking (in West Circassian and Kabardian). Morphemes demonstrate features that are not typical of morphemes in Standard Average European languages, including a high degree of autonomy reflected in affix order variation, widespread morphological recursion, occasional reduplication and even the ability to attach to complex syntactic constituents.