The dimensional synthesis of spatial chains for a prescribed set of positions can be used for the design of parallel robots by joining the solutions of each serial chain at the end effector. In some cases, this may yield a system with negative mobility. The synthesis of some overconstrained but movable linkages can be done by comparing the known screw system associated to the motion of the linkage to that generated by the tasks positions defined by the synthesis. This paper present the simplest case, that of the spatial RPRP closed chain, for which one solution exists.