Block symmetry in discrete memoryless channels

Various notions of block symmetry for discrete, memoryless channels are introduced. Though block symmetry in itself does not guarantee easy calculation of channel capacity and easy determination of optimal distributions, usually, it does simplify matters, and also, known results in this direction – often based on rather strong concepts of symmetry – appear as simple corollaries to the results here presented. The findings appear to be relatively simple and natural ones which somehow were not considered or overlooked in the early development of information theory