Documentation of the language of this period is scanty, making it difficult at best fully to determine the relationship between the literary language and its spoken dialects.There are references in Arab and Byzantine sources to pre-Christian Slavs in European Russia using some form of writing.Despite some suggestive archaeological finds and a corroboration by the tenth-century monk Chernorizets Hrabar that ancient Slavs wrote in "strokes and incisions", the exact nature of this system is unknown.
(sic, with one ‘s’, from Rus’), was a language used in the 10th–15th centuries by East Slavs in Kievan Rus' and states which evolved after the collapse of Kievan Rus'.
Dialects of it were spoken, though not exclusively, in the area today occupied by Belarus, central and northern Ukraine, and parts of western Russia. The Russian drevnerusskij jazyk means Old Rus’ian as well.
Rusian is similarly used by some western scholars such as Horace Lunt.
The language was a descendant of the Proto-Slavic language and faithfully retained many of its features.
A striking innovation in the evolution of this language was the development of so-called pleophony (or polnoglasie 'full vocalisation'), which came to differentiate the newly evolving East Slavic from other Slavic dialects.
Other Slavic dialects are differed by resolving the closed-syllable clusters *e RC and *a RC as liquid metathesis (South Slavic and Czech-Slovak), or by no change at all (see the article on Slavic liquid metathesis and pleophony for a detailed account).Since extant written records of the language are sparse, it is difficult to assess the level of its unity.In consideration of the number of tribes and clans that constituted Kievan Rus, it is probable that there were many dialects of Old East Slavonic.Therefore, today we may speak definitively only of the languages of surviving manuscripts, which, according to some interpretations, show regional divergence from the beginning of the historical records.Nonetheless, by 1150 it had more unity than any other branch of Slavic, showing the fewest local variations.With time, it evolved into several more diversified forms, which were the predecessors of the modern Belarusian, Russian, Rusyn and Ukrainian languages.