Meijers, E.-M Keywords: Legal principles | Private international law | Legal history | Mots clefs: Principes de droit | Droit international privé | Histoire du droit | ABSTRACT In this course, E.-M. Meijers proposes to explain how and why the principles of private international law are formed and succeed each other from the Middle Ages until the formation of modern codes. After a study of the regime of the kind of laws, the author studies the conflicts of custom in Western Europe in the Middle Ages, the Italian theory of S…