HEYN pointed out in 1914 that plastic deformation of a metal could result in irregular internal stresses because of the inherent elastic and plastic anisotropy of the grains1. With the development of X-ray diffraction methods it has been possible to study the internal stresses by direct experiment. Such X-ray measurements, made by Wood and his co-workers, led, however, to the unexpected result that the internal stresses in a number of metals were by no means irregular, being related in magnitude and direction to the previously applied stress2. Iron and carbon steels are typical examples. It was concluded that the stresses were too regular to be explained satisfactorily by the type envisaged by Heyn (which we shall term intergranular stresses).