 The book is divided into two parts, one on special relativity and the other on general relativity, each in some respects dependent on the other. The first part presents a series of postulates about the behavior of light, accompanied by a series of definitions. The primitives are a directed acyclic graph whose vertices are point events and whose edges represent the relation “a signal can be sent from event A and received at point B,” and a partitioning of the event space by assignment of each event to a point on some real line—intuitively, a world line—such that the set of events assigned to the same line are equicardinal with the line and their ordering by signaling is the ordering of the real line, and such that no two such world lines intersect. Axioms about the behavior of signals, especially “first signals”—meaning light signals—are intended to specify sufficient constraints so that the coordination determines an inertial frame of reference in Minkowski space-time. Extensive discussion is given over to the impossibility of experimentally determining a “metrical” simultaneity relation—that is, of finding experimentally a unique simultaneity 3-space for an inertially moving observer. “Matter axioms” are then added specifying transport properties of rigid rods and the connection of rigid rod distance measurements with round trip time measurements (although one of the “matter axioms,” Axiom VI.2, is entirely about clock transport). Reichenbach asserts that the four matter axioms that are restricted to inertial frames follow from his light axioms. That is not true, and 60 pages later Reichenbach qualifies the claim to mean only that the relations follow from the general theory of relativity. 