Resemblances between two or more languages (whether in typology or in vocabulary) can be due to genetic relation (descent from a common ancestor language), to borrowing between languages, to retention of features when a population adopts a new language, or simply to chance. When little or no direct documentation of ancestor languages is available, determining whether a similarity is genetic or areal can be difficult. Edward Sapir notably used evidence of contact and diffusion as a negative tool for genetic reconstruction, treating it as a subject in its own right only at the end of his career (e.g., for the influence of Tibetan on Tocharian).
Genetic relationships are represented in the family tree model of language change, and areal relationships are represented in the wave model. Labov in 2007 reconciled these models in a general framework based on differences between children and adults in their language learning ability. Adults do not preserve structural features with sufficient regularity to establish a norm in their community, but children do. Linguistic features are diffused across an area by contacts among adults. Languages branch into dialects and thence into related languages through small changes in the course of children's learning processes which accumulate over generations, and when speech communities do not communicate (frequently) with each other, these cumulative changes diverge. Diffusion of areal features for the most part hinges on low-level phonetic shifts, whereas tree-model transmission includes in addition structural factors such as "grammatical conditioning, word boundaries, and the systemic relations that drive chain shifting".
In some areas with high linguistic diversity, a number of areal features have spread across a set of languages to form a sprachbund (also known as a linguistic area, convergence area or diffusion area). Some examples are the Balkan sprachbund, the Mainland Southeast Asia linguistic area, and the languages of the Indian subcontinent.
- The use of the plural pronoun as a polite word for you in much of Europe (the tu-vous distinction).
- The spread of the guttural R from either German or French to several West European languages.
- The tendency to use a habeo (transitive, e.g. "I have") construction for possession in much of Europe, instead of a mihi est (to me is) construction, which is more likely the original possessive construction in Proto-Indo-European, considering the lack of a common root for "have" verbs.
- The development of a perfect aspect using "have" + past participle in many European languages (Romance, Germanic, etc.).
- A perfect aspect using "be" + past participle for intransitive and reflexive verbs (with participle agreement), present in French, Italian, German, older Spanish and Portuguese, and possibly even English, in phrases like "I am become[ death, destroyer of worlds]" and "The kingdom of this world is become".
- Presence of /ɫ/ (dark L), usually contrasting with palatalized /lʲ/ in Slavic, Baltic and Turkic languages of Eastern Europe.
- Possibly the Satem sound change.
- Postposed article, avoidance of the infinitive, merging of genitive and dative, and superessive number formation in some languages of the Balkans.
- Development of a three-tone system with no tones in words ending in -p, -t, -k, followed by a tone split; many other phonetic similarities; a system of classifiers/measure words; etc. in the Mainland Southeast Asia linguistic area.
- Retroflex consonants in the Burushaski, Nuristani, Dravidian, Munda, and Indo-Aryan families of the Indian subcontinent.
- The occurrence of click consonants in Bantu languages of southern Africa, which originated in the Khoisan languages.
- The lack of fricatives in Australian languages.
- The use of ejective and aspirated consonants in the languages of the Caucasus.
- The spread of a verb-final word order to the Austronesian languages of New Guinea.
- The prevalence of ejective and lateral fricatives and affricates in the Pacific Northwest of North America.
- The development of a close front rounded vowel in the Bearnese dialect of Occitan and the Souletin dialect of Basque.
- The absence of [w] and presence of [v] in many languages of Central and Eastern Europe.
- The lack of nasal consonants in languages of the Puget Sound and the Olympic Peninsula.
- Drechsel, Emanuel J. (1988). "Wilhelm von Humboldt and Edward Sapir: analogies and homologies in their linguistic thoughts", in Shipley, William (ed.) (December 1988). In Honor of Mary Haas: From the Haas Festival Conference on Native American Linguistics. the Hague: de Gruyter Mouton. p. 826. ISBN 978-3-11-011165-1. p. 254.
- Labov, William (2007). "Transmission and diffusion" (PDF). Language. Baltimore: LSA. 83: 344–387. doi:10.1353/lan.2007.0082. Retrieved 18 Aug 2010.
- Labov 2007:6.
- Winfred Philipp Lehmann, Historical Linguistics: An Introduction, Routledge, 1992, p. 170
- Berger, H. Die Burushaski-Sprache von Hunza und Nagar. Vols. I-III. Wiesbaden: Harrassowitz 1988
- Tikkanen (2005)
- G. Morgenstierne, Irano-Dardica. Wiesbaden 1973
- The Munda Languages. Edited by Gregory D. S. Anderson. London and New York: Routledge (Routledge Language Family Series), 2008. ISBN 978-0-415-32890-6
- Campbell, Lyle (2006). "Areal linguistics: A closer scrutiny". In Matras, Yaron; McMahon, April; Vincent, Nigel. Linguistic areas: Convergence in historical and typological perspective. Basingstoke: Palgrave Macmillan. pp. 1–31.
- Campbell, Lyle (2006). "Areal linguistics". In Brown, Keith. Encyclopedia of language and linguistics (2nd ed.). Oxford: Elsevier. pp. 1.455–460.
- Haas, Mary R. (1978). Language, culture, and history, essays by Mary R. Haas, selected and introduced by Anwar S. Dil. Stanford: Stanford University Press.
- Haas, Mary R. (June 1978). Prehistory of Languages. The Hague: de Gruyter Mouton. p. 120. ISBN 978-90-279-0681-6.
- Hickey, Raymond (ed.) (2017). The Cambridge Handbook of Areal Linguistics. Cambridge: Cambridge University Press.