Abstract.Sufficient conditions under which principal directions of stress and strain rate must coincide are established rigorously. It is the coincidence of these directions which permits a proper interpretation of principal strain rate components in principal stress space.Introduction. A rigid perfectly plastic solid is characterized by a "yield" or "limit" surface /( where a',-is the stress deviator tensor) then normality requires that for some scalar X, ei; = X(3//3o-i,) = XaJ,-; coincidence of principal directions of stress and strain rate is then immediate since the principal axes of the stress and stress deviator tensors coincide.More generally, if / is a function of stress invariants which is differentiable in the components of cr,, then the principal directions of cr,,-and d//d cr2 > c3 . Deformation occurs by simple shearing in the direction of the shear stress vector T if the magnitude r of T reaches a critical value k. Since the planes of maximum shearing, called slip planes, are orthogonal, the simple