Why can an investor today automatically acquire rights from a treaty its home State never signed? Arbitral tribunals and scholars have assumed the answer is obvious. In this work, you will see that it is not. Drawing on nine centuries of treaty practice — from medieval trade privileges to the ICJ and modern arbitration case law — this book reveals that the most-favoured-nation clause granted States a right to claim, not an automatic extension of rights. That conceptual leap emerged from the litigation needs of the 1950s. If you have to interpret MFN clauses, be it for litigation or study purposes, this book changes what you think they mean.