The essay contains six main departments. The first examines the sound composition of the common Slavic proto-language. The second section sets out the sound phenomena that took place in the era of the general life of the southern and eastern Slavs, according to the separation of the western branch of the common Slavic language family from it. The third department lists the phenomena that arose in the era of the joint life of all Russian tribes. The fourth department sums up the results by depicting the sound composition of the Old Russian language. In the fifth department, the fall of semi-short sounds, which occurred no earlier than the XII century, is investigated. The sixth department sets out the oldest dialectical phenomena of the Russian language and determines the main sound differences of all three Old Russian dialects.