Among the many tensions and oppositions in play in the early twentieth century, one—the divide between classical and modern physics—has retrospectively overshadowed our understandings of the period. This paper investigates when and why physicists first started using the term ‘classical’ to describe their discipline. Beginning with Boltzmann and ending with the 1911 Solvay Congress, on a broad scale this story constitutes a powerful instance of the circulation of a rich cultural image. First deployed in understandings of literature, music, art and (...) schooling, the concept of the classical within the physics community came to be invested with a highly specific meaning, which in turn formed the basis for the widespread popularization of a new physical worldview after World War I. But on a finer scale, displaying the diverse, contrasting and controversial concepts of classical theory invoked by different physicists around 1900, and charting the emergence of our present understanding with the rise of relativity and quantum theory, reveals significant tussles over the meaning and value of different intellectual approaches. Here I use these tensions to investigate the interrelations between research programs and the broader, framing concepts with which physicists describe their experience of disciplinary change.Keywords: Energetics; Statistical mechanics; Relativity; Quantum theory; Classical physics; Ludwig Boltzmann. (shrink)