Abstract2.1 Plücker Vector2.2 Rigid Body’s Motion Expression2.3 Screw Expression of Motion and Force2.4 Reciprocal Product of Screws and its Geometric Meaning2.5 Linear Combinations of Screws and Principal Screws of a Screw System2.6 Identification of Principal Screws of a Screw System2.7 ConclusionsReferences