The dynamic behavior of a linear, undamped, vibratory system is characterized by the solution of the symmetric definite eigenvalue problem Ax = λBx. Lord Rayleigh investigated the results of modifying the system in the special cases in which either the kinetic energy of the system is changed leaving the potential energy unchanged or, conversely, the modification changes the potential energy without altering the kinetic energy of the system. We consider here the dynamic consequences of modifying the system by adding elements which change both the system's kinetic and potential energy, simultaneously. © 1991.