The elastic behaviour of multistoried structures acted on by lateral forces, including seismic forces, may be characterized by means of the concept of “level relative rigidity”. The relative rigidity between the consecutive levels is of fundamental significance for the analysis and design of earthquake resistant sturctures. The importance of the story rigidity is noticeable in many of its applications such as the aseismic design of structures, the determination of the natural periods of vibration, the distribution of the basic sbearing force along tbe beigbt of tbe structure, tbe distribution of the story seismic force between the resistant members of tbe respective story, the computation of the borizontal displacements of the structrure. Tbe whole theory of free and forced oscillations of structures may be stated in terms of the above quantities. In general, it may be said that with the help of the matrix of relative rigidities, the dynamic respons~ of structures acted on by lateral forces caused by seismic motion, wind or explosions, is fully determined. In the following we shall refer only to the elastic behaviour of multistoried frames acted on by the lateral forces due to earthquake motion.