Schematic of subunit packing and interfaces in a B-type fibril. (a and b) Cross section and side view of a B-type fibril. (c and d) Schematic representation of the dimer-of-dimers subunit packing for the same fibril orientation; each elliptical cylinder corresponds to two β2m monomers. The outermost (orange) protofilaments in the model are disordered in the map, so that their full density is not reconstructed. It is possible, although unlikely, that pairs of protein subunits are threaded through all three protofilaments in one crescent with flexible hinge regions between the protofilaments.