User blog comment:Cask Of Armadillo/Modern Slavlang Challenge/@comment-25410520-20150927021042/@comment-26873416-20150927021439

Well, a language descended from Old East Slavic will be mutually intelligible with Russian and Ukrainian. If it's from Old Church Slavonic then it will be mutually intelligible with all the Southeastern Slavic languages. This rule serves to make the languages more realistic.