A general method for the computation of the canonical form of three-systems of infinitesimal screws is presented. The method is particularly simple when the three-system has a basis that is simultaneously perpendicular and reciprocal. However, it can also handle the special or degenerate cases. The method and the concurrent results are important from the theoretical point of view because of the obvious connection with the classification of
screw-systems. Moreover, the results are also important for applied kinematics after recent applications of the canonical form of screw-systems to the analysis and synthesis of manipulators and manipulator substructures.