Considerations of the electron channelling pattern formation in a medium consisting of subcells with orientations oscillating in a range, α±δα, are presented. These are followed by an experiment which compares well with theoretical expectations. It is shown that selected area channelling pattern (SACP) line resolution not only depends on the total density of dislocations but also on their distribution. It is also shown that for a given dislocation density, the degradation of SACPs is more pronounced for a random distribution than for one with a subcell structure. The arrangement of the dislocations into subcells make the body a two-component medium in which the undeformed spaces are penetrated by a high density dislocation array forming a three-dimensional network of subcell walls. This kind of structure will generate an SACP which is a superposition of relatively undeformed channelling patterns. These are shifted each from the other by an angle α which is a function of subcell misorientation. The increase in the width of SACP-lines is mostly influenced by the value of ±δα which can be considered as a scattering band for an average subcell misorientation typical for a given dislocation density.