The article examines the usage of graphemes ѹ, ю, ѫ, ѭ in Il’ina kniga (Russian State Archive, F.381, No. 131). Regularities of using these graphemes provide evidence for the existence of certain spelling rules. For example, scribe Ilya never uses ѭ after consonants (in this position only ю is used), and ѫ very seldom appears at the beginning
of words or after vowels. On the whole Ilya preferred to write ѫ in roots and the digraph оу in inflections. He quite consistently associated some roots with grapheme ѫ, and ѫ-marking could extend to homophonous roots (thus, under the influence of the root лѫк- the words прилоучитися, оулоучити, лѹча and the comparative лѹчии were
written in most cases with jus).