  During the next eight years the binary relation was reinvented by a number of logicians. Prior, in his address to a conference in Wellington in 1954, seems to have been the first to use the binary relation in an explicitly tense-modal context. Other landmarks were an address by Montague to a conference held at UCLA in 1955 (Montague’s binary relation held between models rather than points or indices interpretable as worlds, and the notion of a possible world was absent), Prior and Meredith’s property calculus of 1956, lectures by Smiley in Cambridge in 1957 (Smiley pursued an algebraic approach), Kanger (1957), Hintikka (1957, 1961) and Kripke (1959a, 1959b, 1963). Kripke was familiar with Kanger’s work involving the binary relation at the time he obtained his own results. Kanger himself had read the 1951 paper by Jonsson and Tarski and he described his results as similar to theirs (Kanger 1957: 39). (Copeland (2002) gives a detailed history of possible worlds semantics.)