The elusive etiology of germline bias of the T cell receptor (TCR) for major histocompatibility complex (MHC) has been clarified by recent'proof-of-concept'structural results demonstrating the conservation of specific TCR-MHC interfacial contacts in complexes bearing common variable segments and MHC allotypes. We suggest that each TCR variable-region gene product engages each type of MHC through a'menu'of structurally coded recognition motifs that have arisen through coevolution. The requirement for MHC-restricted T cell recognition during thymic selection and peripheral surveillance has necessitated the existence of such a coded recognition system. Given these findings, a reconsideration of the TCR–peptide-MHC structural database shows that not only have the answers been there all along but also they were predictable by the first principles of physical chemistry.