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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.
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- 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 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.)
- 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 European languages east of France
- the lack of nasal consonants in languages of the Puget Sound and the Olympic Peninsula
- Linguistic typology
- World Atlas of Language Structures
- Comparative method
- Mass lexical comparison
- Language contact
- Linkage (linguistics)
- Wave model
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- Labov, William (2007). "Transmission and diffusion" (PDF). Language (Baltimore: LSA) 83: 344–387. doi:10.1353/lan.2007.0082. Retrieved 18 Aug 2010.
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- Campbell, Lyle. (In press). Areal linguistics. In K. Brown (Ed.), Encyclopedia of language and linguistics (2nd ed.). Oxford: Elsevier. (Online version: http://www.linguistics.utah.edu/Faculty/oldFacultyPages/campbell/CampbellArealLingEnc.doc).
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