The main strategic international partner of the portal Historical Correspondence Online is the interdisciplinary research project Cultures of Knowledge at the University of Oxford funded by the Andrew W. Mellon Foundation and the Arts and Humanities Research Council. The project uses digital methods with the aim of gathering and interpreting Early Modern Period correspondence of predominantly scholarly provenance. For this purpose it has been building the extensive database Early Modern Letters Online (EMLO), which is continually expanding and currently (February 2021) includes more than 164,000 letters. Conceptually and from the technological point of view, Historical Correspondence Online is a follow-on from the long-term cooperation of the Institute of Philosophy at the Czech Academy of Sciences with the project Cultures of Knowledge and with the EMLO database. We regularly consult our Oxford colleagues on the development of the portal, especially with regard to the maintenance of compatibility with the EMLO database and the possible future integration of data into the European infrastructure. EMLO shares with HIKO correspondence data of Jan Amos Comenius, Philipp Jakob Sachs von Löwenheim, Amandus Polanus of Polansdorf and Czech students at Protestant universities. These catalogues are further enriched and enlarged with the new data in HIKO.