Ranking and Comparing Ligands. As described clearly in BB 2013, application of all eight rules involves the same two processes: Ligands are ranked sphere by sphere, branch by branch in a breadth-first fashion. Then, in pairs, two ligands are compared atom by atom, in order of that ranking. In practice, it is not always necessary to completely rank a ligand, including hydrogen and phantom atoms. Rankingat least through Rule 2can be carried out on a "need-to-know" basis, skipping whole sub-branches of the digraph where a decision has already been made.