isogloss
A boundary that separates regions in which different languages usages predominate.
An isogloss is a geographical boundary line that separates two or more regions or linguistic communities where the same language or dialect spoken varies slightly. It typically marks the area where different linguistic characteristics, such as pronunciation, vocabulary, or grammar, begin or end. The boundary lines can vary in size, depending on how significant the differences between the langauage varieties are. For example, an isogloss line may separate regions where the same language is spoken but with different accent or vocabulary or it can separate completely different language systems. Isoglosses are important for mapping the distribution of language variation around the world and for understanding the historical linguistic processes that shape regional dialects and languages.
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