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The role of a river as a boundary or border in the Middle Ages: The Meuse, from Explaining the role of the Middle Meuse, from Sedan to Maastricht, with reference to the spatial definition used by the authorities of the Roman period in the 16th century, proved difficult until the 12th century. It is therefore necessary to give a geographical definition of the concepts of boundary and border and to make a few methodological comments regarding this issue. When borders become fixed, it can be observed that the river ceases to be a theoretical spatial reference point but instead becomes a very real one. When the Meuse becomes a border, it operates as such for navigation at least within its navigable course. But it seems impossible to assign to the river a systematic function as regards demarcation lines between different local powers. It is also important to stress the constant interest these powers have shown in the river.