A program called XpY was written for the PDP-10/LDS-1 at the Princeton University Computer Graphics Laboratory, for generating and displaying models of dinucleoside phosphates. The molecule GpC, a member of this class and a fragment of the nucleic acid RNA, was subjected to X-ray diffraction analysis.The paper describes the importance of model building in X-ray analysis, and shows step by step how XpY was used to deduce the atomic coordinates of GpC from the experimental data. The program documentation is also included as an Appendix.A subjective critique of graphics is made in the Conclusions section.