The East Slavic languages constitute one of three regional subgroups of Slavic languages, currently spoken in Eastern Europe. It is the group with the largest numbers of speakers, far out-numbering the Western and Southern Slavic groups. Current East Slavic languages are Belarusian, Russian, Ukrainian, and Rusyn (a small language spoken in Eastern Slovakia, South Eastern Poland, Eastern Hungary and South Western Ukraine and regarded by many as a Ukrainian dialect).
All these languages are nowadays considered to be separate languages in their own right, though in the 19th century it was usual to call Ukrainian ("Little Russian") and Belarusian ("White Russian") dialects of one common "Russian" language (the most prestigious dialect of which was called "Great Russian"). Despite the vast territory occupied by the East Slavs, their languages are astonishingly similar to one another, with transitional dialects in border regions.
All these languages use the Cyrillic alphabet, but with particular modifications.
When the common Old East Slavic language became separated from the ancient Slavic tongue common to all Slavs is difficult to ascertain (6th–11th century).
The history of the East Slavic languages is a very 'hot' subject (see the discussion on this article), because it is interpreted from various political perspectives by the East Slavs "like all mortals, wishing to have an origin as ancient as possible" ("sicut ceteri mortalium, originem suam quam vetustissimam ostendere cupientes"), as Aeneas Sylvius observed in his Historia Bohemica in 1458.
Therefore, a crucial differentiation has to be made between the history of the East Slavic dialects and that of the literary languages employed by the Eastern Slavs. Although most ancient texts betray the dialect their author(s) and/or scribe(s) spoke, it is also clearly visible that they tried to write in a language different from their dialects and to avoid those mistakes that enable us nowadays to locate them.
In both cases one has to keep in mind that the history of the East Slavic languages is of course a history of written texts. We do not know how the writers of the preserved texts would have spoken in every-day life, let alone how an illiterate East Slavic peasant spoke to his family.
History of the literary languages
What follows is a short overview over the Old and Middle periods. For more detail see Old East Slavic language, Ruthenian language, and History of the Russian language.
After the conversion of the East Slavic region to Christianity the people used service books borrowed from Bulgaria, which were written in "Old Bulgarian" or Old Church Slavonic. They continued to use this language, or rather a variant thereof, usually called (Middle) Church Slavonic, not only in liturgy, but also generally as the language of learning and written communication. This left a large imprint even on the rare secular texts.
Throughout the Middle Ages (and in some way up to the present day) there existed a duality between the Church Slavonic language used as some kind of 'higher' register (not only) in religious texts and the popular tongue used as a 'lower' register for secular texts. It has been suggested to describe this situation as diglossia, although there do exist mixed texts where it is sometimes very hard to determine why a given author used a popular or a Church Slavonic form in a given context.
History of the dialects
The first divergence among the Old East Slavic texts is evident during the 12th century, during the era of Kievan Rus', i.e. some texts can be linguistically located to areas that are now in Russia, Ukraine or Belarus. This leads many Russian scholars to speak of the existence of a separate Russian language as early as the 12th century.