The article examines the contribution in the musical science of two out­standing musicians-theorists: Dmitry Cantemir and Grigor Gapasakalian, who work­ed in the Ottoman Empire in the 17th-18th centuries. The Book on the Science of Music belongs to scientist-encyclopedist D. Kantemir. It is a musical-theoretical treatise dedicated to the theory of Turkish classical music, in which Kantemir appears as the author of a remarkable musical notation based on the Arabic alphabet. Armenian musician-theorist Grigor Gapasakalian proposed in his main work The Book of Music, music notation based on elements of post-By­zantine (Greek), medieval Armenian and Eastern musical semiographies, the purpose of which was to overcome the crisis of declining neumatic (xaz) notation. Both figures who worked in Constantinople, which was the center of the musical traditions of various confessions, made a significant contribution to the de­ve­lop­ment of musical, theoretical and aesthetic thought in the Middle East.