Compiling Horn-Clause Rules in IBM's Business System 12 - an Early Experiment in Declarativeness Ghica van Emde Boas-Lubsen, Peter van Emde Boas Abstract: Compiling Horn­Clause Rules in IBM's Business System 12: an Early Experiment in Declarativeness Ghica van Emde Boas­Lubsen, Peter van Emde Boas The tight connection which exists between the fragment of Prolog now known by the name Datalog [28] and the various calculi and algebras for Relational Database Systems was observed at several places in the late 70­ies and early 80­ies. The problem was to make this idea operational and to build a system which implemented it. Such systems today are known as Deductive Databases. We describe the history of a hardly known project from the mid 80­ies where a prototype realizing this goal was produced. We explain why the Relational Database system called Business System 12, developed by IBM in the Netherlands, and which became operational in 1983, turned out to provide the right functionality. We also indicate how this project influenced subsequent projects aimed at enhancing the degree of declarativeness in interfaces with database systems. "What we understand we can formalize; what is formalizable can be automated, and you will discover that someone has built it."