The principles governing the molecular design of collagen have been investigated using molecular graphics to link the available sequence and X-ray diffraction data. The primary structure is classified into three regions of relevance to the intermolecular packing. Functionally important variations are revealed in the distribution of the amino acid residues. Solvent accessibility studies show that the formation of the triple helix is comparable to the stage of secondary structure formation in globular proteins. The remaining hydrophobic interaction potential serves to determine the three-dimensional packing of the molecules within a fibril. A model for the fibril compatible with the X-ray data is suggested. Particular attention is directed onto the boundaries of the overlap region designated the T zones. Azimuthal orientations of the helical portions at these levels appear to be locked by means of a network of aromatic interactions.