“…Of course, this evidence does not specify the conformation in detail, but it was possible to devise two independent mathematical model-building methods that start from the known covalent structure and search for sterically possible ways in which the chain might be draped to give a double helix of this sort having the dimensions and symmetry required by the X-ray-diffraction evidence. It was satisfying that both methods led to essentially the same double helix (Anderson etal., 1969); a projection and photographs of this have been published (Anderson et al, 1969;Rees et al, 1969;Rees, 1969bRees, , 1970b. When a molecular model was actually set up we were delighted to see that both hydroxyl groups in the repeating unit [see structure (II)] were positioned to engage in hydrogen-bonding between the stands.…”