Two theories have been put forward to account for the broadening of the X-ray reflexions given by cold-worked metals. First, the crystals may be broken down into small mosaic elements (fragmentation)1, or, secondly, they may be so distorted by the stresses (microstresses) present that the lattice parameter varies over a range of values2. Recent work seems to indicate definitely that in the case of copper3, tungsten and α-brass4 the major part of the broadening is due to the latter effect.