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The paper considers the decoupling problem, i.e. the identification of the dynamic behaviour of a structural subsystem, starting from the known dynamic behaviour of the coupled system, and from information about the remaining part of the structural system (residual subsystem). Substructure decoupling techniques can be classified as inverse coupling techniques or direct decoupling techniques. In inverse coupling, the equations written for the coupling problem are rearranged to isolate (as unknown) one of the substructures instead of the coupled structure. Examples of inverse coupling are impedance and mobility approaches. Direct decoupling consists in adding to the coupled system a fictitious subsystem which is the negative of the residual subsystem. Starting from the 3-field formulation (dynamic balance, compatibility and equilibrium at the interface), the problem can be solved in a primal or in a dual manner. Compatibility and equilibrium can be required either at coupling DoFs only, or at additional internal DoFs of the residual subsystem. Furthermore DoFs used to enforce equilibrium might be not the same as DoFs used for compatibility: this generates the so called non collocated approach. In this paper, direct decoupling techniques are considered: primal and dual formulation are compared in combination with collocated and non collocated interface