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In linguistics, a word stem is a part of a word responsible for its lexical meaning. The term is used with slightly different meanings depending on the morphology of the language in question. In Athabaskan linguistics, for example, a verb stem is a root that cannot appear on its own and that carries the tone of the word. Athabaskan verbs typically have two stems in this analysis, each preceded by prefixes.
In most cases, a word stem is not modified during its declension, while in some languages it can be modified (apophony) according to certain morphological rules or peculiarities, such as sandhi. For example in Polish: miast-o ("city"), but w mieść-e ("in the city"). In English: "sing", "sang", "sung".
Uncovering and analyzing cognation between word stems and roots within and across languages has allowed comparative philology and comparative linguistics to determine the history of languages and language families.