International Code of Zoological Nomenclature
In regulating the names of animals it holds by six central principles, which were first set out (as principles) in the third edition of the code (1985):
In botanical nomenclature, the equivalent for "binominal nomenclature" is "binary nomenclature" (or sometimes "binomial nomenclature").
There are approximately 2-3 million cases of this kind for which this principle is applied in zoology.
In the family-group, publication of the name of a family, subfamily, superfamily (or any other such rank) also establishes the names in all the other ranks in the family group (family Giraffidae, superfamily Giraffoidea, subfamily Giraffinae).
Author citations for such names (for example a subgenus) are the same as for the name actually published (for example a genus). It is immaterial if there is an actual taxon to which the automatically established name applies; if ever such a taxon is recognised, there is a name available for it.
Genera are homonyms only if exactly the same — a one-letter difference is enough to distinguish them.
In species, there is a difference between primary and secondary homonyms. There can also be double homonyms (same genus and species). A slight difference in spelling is tolerated if Article 58 applies.
Primary homonyms are those with the same genus and same species in their original combination. The difference between a primary junior homonym and a subsequent use of a name is undefined, but it is commonly accepted that if the name referred to another species or form, and if there is in addition no evidence the author knew that the name was previously used, it is considered as a junior homonym.
Article 59.3 states that in exceptional cases, junior secondary homonyms replaced before 1961 by substitute names can become invalid, "...unless the substitute name is not in use," an exception of the exception. However, the ICZN Code does not give an example for such a case. It seems that this passage in the ICZN Code is widely ignored. It also does not define what the expression "is not in use" should mean.
Double homonymy (genus and species) is no homonymy: if the genera are homonyms and belong to different animal groups, the same specific names can be used in both groups.
For disambiguating one genus-group name from its homonym, it is important to cite author and year. Citing the author alone is often not sufficient.
In some cases, the same genus-group or species-group name was published in the same year by the same author. In these cases it is useful to cite the page where the name was established.
There are cases where two homonyms were established by the same author in the same year on the same page:
For names above the family level, the principle of homonymy does not apply.
Homonyms occur relatively rarely in families (only if generic names are identical or very similar and adding an ending "-idae" produces identical results). Discovering such a homonymy usually produces the same problems as if there were no rules: conflicts between entirely independent and unconnected groups of taxonomists working in different animal groups. Very often the Commission must be asked to take a decision.
This is the principle that each nominal taxon in the family group, genus group, or species group has—actually or potentially—a name-bearing type fixed that provides the objective standard of reference that determines what the name applies to.
Designation and fixation have different meanings. A designation is the proposal of the type species. It is not necessary to have spelled the name of the genus or species correctly with correct authors (articles 67.2.1, 67.6, 67.7), type species are always the correctly spelled name. If the designation is valid, the type species is fixed.
A designation can also be invalid and ineffective—for example—if the genus had already a previously fixed type species, or if a type species was proposed that was not originally included, or contradicted the description or figure for a genus for which no species had originally been included.
A species-group name can have a name-bearing type specimen, but this is not a requirement. In many cases species-group names have no type specimens, or they are lost. In those cases the application of the species-group name is usually based on common acceptance. If there is no common acceptance, there are provisions in the Code to fix a name-bearing type specimen that is binding for users of that name. Fixing such a name-bearing type should only be done if this is taxonomically necessary (articles 74.7.3, 75.2, 75.3).
The names above the family group are regulated only as to the requirements for publication; there is no restriction to the number of ranks and the use of names is not restricted by priority.
The rules in the code apply to all users of zoological names. However, its provisions can be interpreted, waived, or modified in their application to a particular case when strict adherence would cause confusion. Such exceptions are not made by an individual scientist, no matter how well-respected within the field, but only by the , acting on behalf of all zoologists. The commission takes such action in response to proposals submitted to it.
The current (fourth edition) code is cited in scientific papers as ICZN (1999) and in reference lists as:-
ICZN 1999. International Code of Zoological Nomenclature. Fourth Edition. The International Trust for Zoological Nomenclature, London, UK. 306 pp.