The aim of this contribution is to present a new approach to the study of the elites of power, and the legal and political history of the Collateral Council in Flanders, which embrace from the Reconciliation of the wallonian provinces with Philip II (1579) until the Twelve Years´ Truce (1609). At the first part, the internal structures of the three Collateral Councils are treated, as well as – with the base of circa 100 biographies – the kind of people that served on them. The second part studies the jurisdictional and real competences of these Councils, in the framework of a non-written rule of law, where the King was the source of every new law that did not know the trias politica. Even, the procedures of decission making at the framework of every Council are studied. Thanks to this, and following a chronological order, the different positions that the Councils defended, in a conjuncture of civil war, are described, as well as their influence at the decission making in questions related with internal and external affairs.