IF a polycrystalline metal is being extended plastically, the tensile stress in any grain in the metal depends on the orientation of the grain and of its neighbours. When the applied stress is removed, the interactions between grains and their neighbours will result in a complex system of Heyn stresses, the residual stress in any grain being of the same order as the difference in the yield tension of the grain and the average yield tension of its neighbours. The important difference between this concept and that advanced by Wood and Dewsnap is that the latter consider the principal cause of the Heyn stresses to be the variation of the strength of the neighbours, whereas here it is considered that the yield tension of the grain itself is equally important.