Flat bands cannot be accidental. Physically, it implies that the itinerancy of the system is not sufficient. Formal reasons are given by a molecular orbital (MO) representation which we are proposing. This MO representation is compactly reviewed and applied for random systems. It has been appeared in Ann. Physics, “Revisiting Flat bands and localization” by Y. Hatsugai. See also arXiv:2104.01236.