 One puzzle about Reichenbach’s view of conventions is why characterizing them remained important to him, since they are, in his mature view, only a feature of the reconstruction of a theory, not an intrinsic logical or semantical feature of any proposition. In principle, separating out different presentations of the same theory and recognizing equivalence amid diverse conventions might be of value, but except in cases such as simultaneity and gravitational theories with extra “absolute forces,” Reichenbach did not use reconstruction to that effect, and the most important question of equivalence in the physics of his era, the relation of wave and matrix mechanics, was solved in a quite different way by John von Neumann (1932). Reichenbach’s emphasis on locating conventions seems instead to be negatively motivated, a continuing prophylactic against claims that various principles are a priori. 