 Between the publication of The Theory of Relativity and A Priori Knowledge, in 1920, and the appearance of The Axiomatization of the Theory of Relativity in 1924, Reichenbach published a series of professional and popular essays expounding or defending the special and general theories and Reichenbach’s own axiomatization, which was published in outline in 1921 in Physikalische Zeitschrift (1921d). The professional essays were variously addressed to audiences of philosophers or physicists. They are interesting for our purposes chiefly because they state philosophical positions that Reichenbach had previously rejected, or would later come to reject, or because they offer interpretations of the theories that are problematic. Notable in the last respect is Reichenbach’s view that general covariance is a substantive claim of general relativity: the laws of physics have the same “form” in every coordinate system and frame of reference. Reichenbach does not explain the idea of the “form” of a physical law. 