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How are Slavic languages divided into East Slavic, West Slavic, and South Slavic branches?

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Map of Europe showing the geographic distribution of Slavic languages, color-coded by language group: East Slavic (Russian, Ukrainian, Belarusian), West Slavic (Polish, Kashubian, Czech, Slovak, Sorbian), and South Slavic (Bosnian-Croatian-Montenegrin-Serbian, Slovene, Bulgarian, Macedonian). Striped areas indicate regions where bilingualism is common. — Slavic languages Distribution of the Slavic languages in Europe.

The Slavic languages are traditionally classified into three distinct branches based on their geographic and linguistic development. While this "family tree" model is a useful organizational tool, it is important to note that historical development was actually a complex process of continuous dialectal differentiation and reintegration rather than a simple split.1

The three branches are divided as follows:

South Slavic: This branch includes two primary subgroups: the Bosnian-Croatian-Montenegrin-Serbian-Slovene group and the Bulgarian-Macedonian group.1
West Slavic: This branch consists of three subgroups: Czech-Slovak, Sorbian, and the Lekhitic languages, which include Polish and related tongues.1
East Slavic: This branch comprises Russian, Ukrainian, and Belarusian.1

Although these branches are categorized separately, linguistic frontiers between spoken dialects are often not sharply defined, and transitional dialects exist that connect the various languages across these traditional boundaries.1