We consider the general problem of root-clustering of a matrix in the complex plane: Let

and

. Find the largest class of

and an algebraic criterion which is necessary and sufficient for
![\\lambda _{i}[A] \\in S, i=1,2,..., n](/images/tex/3666.gif)
. We introduce two types of regions which constitute the largest class of

known to date. The criterion is presented both for open regions and closed ones. The results are used to define a design methodology for control systems. Moreover, all classical results are shown to be special cases of the present theory.