The present chapter addresses the creation process of the fundamental dichotomic system of the modern law of the sea, viz. territorial sea and high seas, in Old Regime Europe thorough the analyses of ‘state practices’ and doctrines regarding dominium maris, based on some preceding historical backgrounds. It also discusses two neglected issues in order to grasp the accurate, at least theoretically, traces in establishing the current perception of ‘territorial sea’ emanating from the ‘cannon-shot rule’, most famously propounded by Bynkershoek for defining the outer limit of the adjacent sea. The discussion concludes that, although the basic notion of the modern dichotomic system had generally been recognised during the epoch surveyed, no unified criterion for determining the extent of the terrestrial authority over the sea had emerged. It suggests, in the end, the importance of two Italians, Galiani and Azuni, as the earliest known theorists who proposed the theoretical equation of the rules of cannon-shot and three-mile limit and indicates the necessity of fathoming the complex spoors of the history of the law of the sea originating from immensely diverse theories and practices.