Five kondakarian manuscripts dating from the 11th to 13th century belong among the oldest witnesses of Russian church music. The Slavonic church texts are accompanied by a peculiar notation which represents the most complex notational system of the European Middle Ages. For centuries this system has been viewed as being undecipherable. In 1962 the author of this book succeeded in deciphering this notation using various methods even though directly compatible Greek sources were unknown at that time. His research appeared in two issues of journal which ceased publication several years ago. After the publication of the first part of this study a Greek philologist discovered in Kastoria (Northern Greece) a unique manuscript with a system of notation similar to that of the Russian kondakaria, confirming there by his research results in a most unexpected and indubitable manner. This path-breaking study has been revised and brought up to date in the present publication. The prominent Russian scholar Yuri Keldysh was of the opinion that "no historian of Russian music would be able to proceed without taking into consideration this study".