The history of the use of dublet letters е – є, о – ѡ, ы – и (after hushing sounds in participles) is analyzed in the article; the analysis is based on the material of the books with editors’ corrections (kavychnye) of the 17th mid-century from the RGADA collection. The change in the usage of these letters, which is based on the antistycherion principle of orthography borrowed from the Greek Language, is described in chronological order. The letters е, ѡ, ы became marked graphemes for the plural. The redundant use of the marked graphemes in the plural was sometimes eliminated in cases where grammatical homonymy was absent. The use of the letter ѡ in prepositions, prefixes and suffixes of adverbs is connected with the differentiation of homonymous morphemes of different parts of speech. The norms of usage of these letters in texts are compared with the rules of the grammars by M.Smotrickiy and F. Polikarpov, as well as with the norms of the modern Church Slavonic language.