A program has been developed to determine the diffraction pattern given by partially ordered fibres formed by macromolecules with helical symmetry. It is particularly useful for visualizing the splitting of layer lines typical of coiled coils. The program produces as output the diffraction diagram calculated for helices that are oriented along their axis but are randomly oriented in other directions. The results can be numerically analyzed and also visualized on-screen. The program has been applied to the diffraction patterns given by DNA and protein coiled coils.