This correspondence is concerned with an image f(B-m) of a box B-m subset of R(m) under a multiaffine transformation f : R(m) --> C. We introduce a notion of a principal point of B-m and prove that the boundary of f(B-m) is covered by images of principal points, We exploit this result to provide necessary and sufficient robust stability conditions for polynomials whose coefficients are multiaffine functions of parameters. An application of the general criterion obtained in the correspondence to the particular case of systems with a cascade of first-order uncertain blocks leads to a computationally tractable procedure that verifies stability of the systems.