 Since Prior’s time, it has become a standard to study tense-logical systems in terms of semantical models based on the idea of branching time. This idea was not realised in Prior’s early works on temporal logic. Indeed it had not yet been formulated in his Time and Modality (1957), which otherwise marked the major breakthrough of the new logic of time. As an explicit (or formalised) idea, branching time was first suggested to Prior in a letter from Saul Kripke in September 1958. This letter contains an initial version of the idea and a system of branching time, although it was not worked out in details. Kripke, who was then only 17 years old, suggested that we may consider the present as a point of Rank 1, and possible events or states at the next moment as points of Rank 2; for every such possible state in turn, there would be various possible future states at the next moment from Rank 3, the set of which could be labelled Rank 4, and so forth. Kripke wrote: