
Balto-Slavic languages include the Baltic languages such as Old Prussian, Lithuanian, and Latvian, and the more numerous Slavic languages such as Russian, Ukrainian, Bulgarian, Polish, and Serbo-Croatian.
Balto-Slavic languages share phonological & morphosyntactic behavior, and lexical content, that make them appear closer than any other set of Indo-European languages save the Indo-Iranian. What is hotly debated is why these languages are so close. The traditional position is that they emerged from a common ancestral tongue called Proto-Balto-Slavic (derived from Proto-Indo-European), which later split into dialects from which developed the separate languages Proto-Baltic and Proto-Slavic. The other position holds that two groups of peoples separately split from the speakers of Proto-Indo-European, one of them coming to speak Proto-Baltic and the other Proto-Slavic; subsequently, extensive contact between these two groups led to their coming to share more of the common linguistic characteristics observed today, while nevertheless maintaining their separate linguistic heritage.