On the Problem of Church Slavonic Writing in the Polish-Lithuanian Commonwealth in the 16–18th Centuries: Religious and Didactic Editions from Two Podlasie Printing Houses. The boundary of Eastern and Western Christianity not only divides Slavs but also fosters the formation of specific space of their contact. For centuries, this space has been represented by Podlasie, which is the borderland of ethnos, languages and confessions. Cultural and spiritual centres of Orthodox Church — Suprasl that possesses unquestionable significance in the history and culture of Podlasie and Zabludov that is also important in some respects — were formed on this border in the 16th century. Historical monuments of writing that were created in Podlasie prove that Church-Slavonic-Ruthenian tradition has become an inseparable part of the common national heritage of the Polish Lithuanian Commonwealth. The 16th–18th centuries Cyrillic writings published in Zabludov and Suprasl make it possible to investigate mutual permeation of elements of Slavia Romana and Slavia Orthodoxa cultures. They are also an important source in studies of the Church-Slavonic language and “prosta mova” that was entering religious texts more and more boldly.