Hierarchy levels of ECOD. Domains placed within the same Architecture share similar secondary structure content (helix, cyan; sheet, yellow) and geometric arrangement. Domains placed within the same X-group share similar structure but lack a convincing argument for homology (vs. analogy), while those placed within the same H-groups are homologous. X- and H- group structures are colored in rainbow by consecutive secondary structure elements. T-groups distinguish homologous domains with notable differences in topology, such as the illustrated Rift-related metafold. Rift-related half-barrels (colored blue and red) are consistent among the domains, but permutations and strand swaps (green) modify the topology.