GENERAL The techniques considered in Item Nos 74019 and 74021 determine whether a given system is stable or not and allow (in the case of graphical methods) the degree of stability to be assessed. However@ more information may be required; for example@ the damping and frequency of particular modes is of interest and diagrams that show how they are affected by the variation of system parameters are useful. The real and imaginary parts of the roots of the characteristic equation@ or the related damping and frequency of the modes@ are plotted as functions of the parameters that enter into the design of the system. Some features of the response of the system are accordingly revealed. The simplest diagram shows the effect of variation in one parameter only and two commonly used forms are considered in Section 3. When the effect of varying two parameters is studied the damping and frequency loci that are drawn only apply to the mode whose stability is of particular interest@ as discussed in Section 4. When the design of a system involves more than two parameters it is usual to treat the variations of one or two of them at a time.