The investigation demonstrated that Haydn did not invent thematic-motivic work as one of the fundamental principles of design in the Viennese Classical period with the string quartets op. 33, which were published in 1781 after a nine-year hiatus, but that this compositional method had already reached a significant level in his work during the 1760s. It became necessary to partially refute or relativize the prevailing opinion, based on the theses of Adolf Sandberger from the early 20th century, regarding the epochal significance of the quartets op. 33. Due to detailed individual studies of Haydn's symphonies created during the quartet hiatus (1773-1781), Sandberger's thesis of a fresh start in 1781 had to be largely modified. The groundbreaking innovation was not the invention of thematic-motivic work itself, but rather the entirely new way in which Haydn employed this technique in op. 33: An important and musically historically promising development was the increasingly close connection between thematic-motivic work and transformations of the theme without losing its recognizability. In this developmental process, the symphonies composed between 1773 and 1781 play a significant role.