Based on the sketchbooks and notebooks from Johann Rudolf Rahn’s stays in Rome and Ravenna (1866/67), his methods of studying an object with his pencil are illuminated in connection with the Swiss art historian’s seldom-discussed early scholarship. In choosing to write his dissertation on “Christian Central Plan Buildings and Dome Construction”, the young Rahn deftly managed to insert himself in the international research discussions in the mid 19th century on the terminological and conceptual separation of Romanesque and Byzantine architecture. His thesis on Rome’s primacy in the development of Christian architecture caught the attention of Carl Schnaase, who invited him to contribute to the new edition of the third volume of his Geschichte der Bildenden Künste in der Schweiz. After being appointed lecturer, Rahn abandoned the “early Christian” line of research around 1870, which also meant the end of his work’s international orientation. The material relating to early Christian and mediaeval art recorded in sketchbooks while in Rome thus remained unused.