St. Petersburg organist, musicologist Boris Samuilovich Kazachkov, in the fourth book about the work of Johann Sebastian Bach, addresses his six organ triosonatas. In the form of an correspondence dialogue with his untimely departed colleague, organist Oleg Vladimirovich Kinyaev (1964-2014), the author sets out assumptions, conjectures and insights both musicians about Bach’s concept of the cycle, its symbolism, offers and compares performance solutions. The text contains a reference to the recording of triosonatas made by B. S. Kazachkov in 2015, in which he captured his artistic ideas.