 Jónsson and Tarski (1951 and 1952) introduce the general idea of Boolean algebras with operators, i.e., extensions of Boolean algebras by addition of operators that correspond to the modal connectives. They prove a general representation theorem for Boolean algebras with operators that extends Stone’s result for Boolean algebras (every Boolean algebra can be represented as a set algebra). This work of Jónsson and Tarski evolved from Tarski’s purely mathematical study of the algebra of relations and includes no reference to modal logic or even logic in general. Jónsson and Tarski’s theorem is a (more general) algebraic analog of Kripke’s later semantic completeness results, yet this was not realized for some time. Not only was Tarski unaware of the connection, but it appears that both Kripke and Lemmon had not read the Jónsson and Tarski papers at the time in which they did their modal work in the late fifties and sixties, and Kripke claims to have reached the same result independently.