The volume focuses on a specific aspect of ratio decidendi: the use by the courts of foreign law as the basis of their decisions when appropriate to the issues to be decided in a particular case brought to them by the litigants. The term foreign law refers to law that is not part of the law binding upon the court, in other words law outside the court’s system of jurisprudence. Thus, one must consider what is domestic law in order to discern what is foreign to, or outside of, it. These comparative essays thus center on what law is foreign in various continental and Anglo-American legal systems from the Middle Ages until the 20th century and how it supports legal arguments and decisions. In this paper, we enlighten a particular case study: the use of foreign law is the practice and jurisprudence of the Parlement of Flanders, a sovereign court established in 1667 by king Louis XIV after the conquest of the southern par of the Spanish Low countries.