One major theme in the philosophy of Hans Reichenbach concerns the presence of elements of convention in human knowledge. The distinction between fact and convention is held to be essential for epistemology and for the philosophy of science (Reichenbach, 1938, Section 1). Indeed, much of Reichenbach's technical work in the philosophy of space and time is addressed precisely to the task of separating out explicitly the conventional component from the empirical core in physical geometry and in the theory of relativity. Thus in the concluding section of (1958) Reichenbach says"…it has been our aim to free the objective core of [statements about reality made by physics] from the subjective additionsintroduced through the arbitrariness in the choice of description." And in the beginning section of his major work in the theory of knowledge (1938), Reichenbach refers to the task of separating fact from convention as constituting "an integral part of the critical task of epistemology." In this paper, I analyze Reichenbach's conventionalism. I shall be especially concerned with the conventionalism attendant upon certain space and time indeterminacies, and, throughout this paper, I shall persist in asking the same question over and over: What is the source of this indeterminacy? As we shall see, there is good reason to raise this question.