Fig. 1
From: Taxallnomy: an extension of NCBI Taxonomy that produces a hierarchically complete taxonomic tree
Evaluation of unranked taxa for rank assignment. Ranked nodes are denoted by filled squares. a In the hypothetical taxonomic tree, node B is between taxonomic-rank level 5 (phylum) which is the lowest rank among its ascendant nodes; and taxonomic-rank level 9 (class) which is the highest taxonomic-rank among all its descendant nodes. Thus, the taxonomic-ranks that node B could assume without affecting the rank hierarchy are subphylum (6), infraphylum (7), or superclass (8), in the balloon. b After evaluating all unranked taxa, we can find (i) those unranked taxa that cannot assume any ranks (node M, since it is between ranked nodes 9, class, and 10, subclass) and (ii) those that could assume one or more ranks (nodes with a balloon)