The orientation behaviour of a range of high-density polyethylenes drawn to different draw ratios has been investigated by measurements of optical birefringence and wide-angle X-ray diffraction (WAXS). Consistent with previous research, it can be concluded that birefringence relates to the overall degree of orientation of the amorphous and crystalline phases, and is described to a good approximation by the pseudo-affine deformation scheme. The WAXS results show that crystalline orientation is also related uniquely to the applied strain and moreover is independent of the characteristics of the polymer concerned. It is shown that the crystalline orientation follows a new model based on the geometry of the crystalline slip mechanism where the crystalline phase deforms affinely in the longitudinal drawing direction.