Historical Slavic languages are a branch of the Indo-European language family, primarily spoken in Eastern Europe and parts of Central Europe. They are divided into three main groups: West Slavic, East Slavic, and South Slavic. Each group includes various languages, such as Polish, Russian, and Bulgarian, which have evolved over centuries from a common ancestral language known as Proto-Slavic. The development of these languages can be traced back to the early medieval period, with significant influences from neighboring cultures and languages. The spread of Christianity in the 9th century, particularly through the work of the Cyril and Methodius brothers, played a crucial role in the standardization and literary development of many Slavic languages.