The principal ranks of taxa in descending sequence are: kingdom (regnum), division or phylum (divisio or phylum), class (classis), order (ordo), family (familia), genus (genus), and species (species). Thus, each species is assignable to a genus, each genus to a family, etc.
Species and subdivisions of genera must be assigned to genera, and infraspecific taxa must be assigned to species, because their names are combinations (Art. 21.1, 23.1, and 24.1), but this provision does not preclude the placement of taxa as incertae sedis with regard to ranks higher than genus.
The genus Haptanthus Goldberg & C. Nelson (in Syst. Bot. 14: 16. 1989) was originally described without being assigned to a family.
The fossil-genus Paradinandra Schönenberger & E. M. Friis (in Amer. J. Bot. 88: 478. 2001) was assigned to “Ericales s.l.” but with respect to family placement it was given as “incertae sedis”.
The principal ranks of hybrid taxa (nothotaxa) are nothogenus and nothospecies. These ranks are the same as genus and species. The prefix “notho” indicates the hybrid character (see App. I).