Many methods and computer programs, e.g. those listed in reference 1, give ‘exact’ initial buckling loads for prismatic structures which consist of thin, rectangular plates which are rigidly connected together along their longitudinal edges. The component plates can be flat or curved, isotropic or anisotropic and can carry any combination of longitudinally invariant in-plane stresses, i.e. longitudinal, transverse and shear stresses. The working which follows can be extended to cover all such plate assemblies, but for simplicity attention is confined to assemblies of uniformly longitudinally compressed, isotropic, flat plates with simply supported ends. The stiffened panel of Fig. 1 is typical of such assemblies, and hence is used to represent them below, because the extrapolation from this particular assembly to the general is obvious.