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This paper aims at reassessing the discussions held in Cologne during the year 1579: they allow to understand the functioning of the Spanish Monarchy in the last decades of the 16th century. Beyond the restitution of the main stages of these negotiations and their results, we would like to apprehend it as a process of social interactions and as a moment when the imperial Spanish project is reorientated. In this way, it becomes possible to make out some of the structures that shaped the Spanish Monarchy. Finally, we intend to put into light the fact that this arbitration was radically different from a formal negotiation as it can be seen after the Peace of Westphalia.