 The study of general models of the axioms of Boolean algebra did not get underway until the late 1920s; it was soon brought to a very high level in the work of Marshall Harvey Stone (1903–1989) (see his papers 1936, 1937). He was interested in the structure of rings of linear operators and realized that the central idempotents, that is, the operators \(E\) that commuted with all other operators in the ring under multiplication (that is, \(EL = LE\) for all \(L\) in the ring) and which were idempotent under multiplication (\(EE = E\)) played an important role. In a natural way, the central idempotents formed a Boolean algebra.