For any taxon in one of the ranks of the family or genus group, the accepted name is the earliest acceptable one that competes at that rank, except in cases of limitation of precedence under Art. 20–24, or where Art. 31 applies.
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For any species-group taxon, the accepted name is the combination of the final epithet of the earliest acceptable name that competes at that rank, with the accepted name of the genus or species to which it is assigned, except (a) in cases of limitation of precedence under Art. 20–24, or (b) if the resulting combination cannot become established under Art. 28.2, or (c) would be unacceptable as a later homonym as defined in Art. 18, or (d) if Art. 31 rules that a different combination be used.
The principle of priority is not mandatory for names of taxa not belonging to the family group, genus group or species group.
Authors should follow the principle of priority also when it is not mandatory, unless the result would be nomenclaturally disruptive and contrary to established tradition.
Once a List has been adopted, all listed names and their co-ordinate names are protected. A protected name is treated as if conserved against earlier homonyms and unlisted competing names; it is treated as established in the place and on the date cited in the list; and its type, when listed, its spelling and, if specified, its gender are treated as if conserved.
Protection can, for individual lists, be restricted with respect to the options set out in Art. 20.2, and particular entries on a list can be exempted from protection. Such restrictions and exceptions are to be specified.
Once a list has been adopted, entries can be added, modified or removed only by the mechanisms of conservation or suppression of names (Art. 21–23). Specified restrictions and exceptions can be waived or modified only upon recommendation of the appropriate committee.
When, for a taxon of the family or genus group, two or more protected names compete, Art. 19.3 governs the choice of name (see also Art. 20.9).
When, for a taxon of the species group, two or more protected names and/or two or more names with the same final epithet and type as a protected name compete, Art. 19.4 governs the choice of name.
The date of protection does not affect the date of a protected name, which is the date of its establishment (Art. 19.1).