The secondary ranks of taxa in descending sequence are tribe (tribus) between family and genus, section (sectio) and series (series) between genus and species, and variety (varietas) and form (forma) below species.
If a greater number of ranks of taxa is desired, the terms for these are made by adding the prefix “sub-” to the terms denoting the principal or secondary ranks. An organism may thus be assigned to taxa of the following ranks (in descending sequence): kingdom (regnum), subkingdom (subregnum), division or phylum (divisio or phylum), subdivision or subphylum (subdivisio or subphylum), class (classis), subclass (subclassis), order (ordo), suborder (subordo), family (familia), subfamily (subfamilia), tribe (tribus), subtribe (subtribus), genus (genus), subgenus (subgenus), section (sectio), subsection (subsectio), series (series), subseries (subseries), species (species), subspecies (subspecies), variety (varietas), subvariety (subvarietas), form (forma), and subform (subforma).
Ranks formed by adding “sub-” to the principal ranks (Art. 3.1) may be formed and used whether or not any secondary ranks (Art. 4.1) are adopted.
Further ranks may also be intercalated or added, provided that confusion or error is not thereby introduced.
The subordinate ranks of nothotaxa are the same as the subordinate ranks of non-hybrid taxa, except that nothogenus is the highest rank permitted (see App. I).
Throughout this Code the phrase “subdivision of a family” refers only to taxa of a rank between family and genus and “subdivision of a genus” refers only to taxa of a rank between genus and species.
For the designation of special categories of organisms used in agriculture, forestry, and horticulture, see Pre. 11 and Art. 28 Notes 2, 4, and 5.
In classifying parasites, especially fungi, authors who do not give specific, subspecific, or varietal value to taxa characterized from a physiological standpoint but scarcely or not at all from a morphological standpoint may distinguish within the species special forms (formae speciales) characterized by their adaptation to different hosts, but the nomenclature of special forms is not governed by the provisions of this Code.