It is worth pointing out that, while the eigenfrequencies of undamped or proportionally damped systems are univocally determined, the eigenvectors are not. In fact, the eigenproblem leaves the scaling factor undetermined. However, it affects only the amplitude, while the shape (that is to say, the relative values of the components of the mode shape vector) remains unchanged. For this reason, conventional scaling procedures are usually adopted to normalize the mode shape vectors. A frequently adopted scaling scheme is based on the orthogonality of natural modes with respect to the mass and stiffness matrices of the structure. The pre- and post-multiplication of those matrices by the modal matrix [①], which collects the mode shape vectors in columns, yield the following diagonal matrices: