The rhombohedral twin boundary of Corundum is used as a model interface for a theoretical case study of codoping and grain-boundary cosegregation of substitutional aliovalent and isovalent cations in Alumina ceramics. This highly ordered interface exemplifies (a) a computational ab initio analysis of electronic structures and total energies by means of the density functional theory, and (b) detailed microscopic insights into fundamental mechanisms of structure and bonding at interfaces related to substitutional cation impurities and dopants.