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. William 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".
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.
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.
^The Munda Languages. Edited by Gregory D. S. Anderson. London and New York: Routledge (Routledge Language Family Series), 2008. ISBN978-0-415-32890-6
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. ISBN978-90-279-0681-6.
Hickey, Raymond (ed.) (2017). The Cambridge Handbook of Areal Linguistics. Cambridge: Cambridge University Press.CS1 maint: Extra text: authors list (link)