Scholars traditionally divide Slavic languages on the basis of geographical and genealogical principle into three main branches, some of which feature subbranches:
- East Slavic
- West Slavic
- South Slavic
Some linguists speculate that a North Slavic branch has existed as well. The Old Novgorod dialect may have reflected some idiosyncrasies of this group.
Mutual intelligibility also plays a role in determining the West, East, and South branches. Speakers of languages within the same branch will in most cases be able to understand each other at least partially, but they are generally unable to across branches (which would be comparable to a native English speaker trying to understand any other Germanic language).
The most obvious differences between the East, West and South Slavic branches are in the orthography of the standard languages: West Slavic languages (and Western South Slavic languages - Croatian and Slovene) are written in the Latin script, and have had more Western European influence due to their proximity and speakers being historically Roman Catholic, whereas the East Slavic and Eastern South Slavic languages are written in Cyrillic and, with Eastern Orthodox or Uniate faith, have had more Greek influence. East Slavic languages such as Russian have, however, during and after Peter the Great's Europeanization campaign, absorbed many words of Latin, French, German, and Italian origin.
The tripartite division of the Slavic languages does not take into account the spoken dialects of each language. Of these, certain so-called transitional dialects and hybrid dialects often bridge the gaps between different languages, showing similarities that do not stand out when comparing Slavic literary (i.e. standard) languages. For example, Slovak (West Slavic) and Ukrainian (East Slavic) are bridged by the Rusyn language/dialect of Eastern Slovakia and Western Ukraine. Similarly, the Croatian Kajkavian dialect is more similar to Slovene than to the standard Croatian language.
Although the Slavic languages diverged from a common proto-language later than any other group of the Indo-European language family, enough differences exist between the various Slavic dialects and languages to make communication between speakers of different Slavic languages difficult. Within the individual Slavic languages, dialects may vary to a lesser degree, as those of Russian, or to a much greater degree, as those of Slovene.