International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature (ICZN or ICZN Code) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. The rules principally regulate:
- How names are correctly established in the frame of binominal nomenclature
- Which name must be used in case of name conflicts
- How scientific literature must cite names
Written nomenclatural rules in zoology were compiled in various countries since the late 1830s, such as Merton's Rules and Strickland's codes going back to 1843. At the first and second International Zoological Congresses (Paris 1889, Moscow 1892) zoologists saw the need to establish commonly accepted international rules for all disciplines and countries to replace conventions and unwritten rules that varied across disciplines, countries, and languages.
Compiling "International Rules on Zoological Nomenclature" was first proposed in 1895 in Leiden (3rd International Congress for Zoology) and officially published in three languages in 1905 (French, English, German; only French was official). From then on, amendments and modifications were subsequently passed by various zoological congresses (Boston 1907, Graz 1910, Monaco 1913, Budapest 1927, Padova 1930, Paris 1948, Copenhagen 1953, and London 1958). These were only published in English, and can only be found in the reports of these congresses or other official publications.
The 1905 Rules became increasingly outdated. They soon sold out, and it became increasingly difficult to obtain to a complete set of the Rules with all amendments. In Copenhagen 1953 the French and English texts of the Rules were declared of equivalent official force, and a declaration was approved to prepare a new compilation of the rules. In 1958, an Editorial Committee in London elaborated a completely new version of the nomenclatural rules, which were finally published as the first edition of the ICZN Code on 9 November 1961.
The 2nd edition of the Code (only weakly modified) came in 1963. The last zoological congress to deal with nomenclatural problems took place in Monte Carlo 1972, since by then the official zoological organs no longer derived power from zoological congresses. The 3rd edition of the Code came out in 1985. The present edition is the 4th edition, effective since 2000. These Code editions were elaborated by editorial committees appointed by the International Commission on Zoological Nomenclature. The ICZN Commission takes its power from a general biological congress (IUBS, International Union of Biological Sciences). The Editorial Committee for the 4th edition was composed of seven persons. Such new editions of the ICZN Code are not democratically approved by those taxonomists who are forced to follow the Code's provisions, neither do taxonomists have the right to vote for the members of the Commission or the Editorial Committee.
As the Commission may alter the Code (by declarations and amendments) without issuing a new edition of the book, the current edition does not necessarily contain the actual provision that applies in a particular case. Currently the Code consists of the original text of the 4th edition and Declaration 44. The Code is published in an English and a French version; both versions are official and equivalent in force, meaning, and authority. This means that if something in the English Code is unclear or its interpretation ambiguous, the French version is decisive, and if there is something unclear in the French Code, the English version is decisive.
The Code deals with zoological nomenclature, which is defined in the Glossary as
- "The system of scientific names for animal taxa and the provisions for the formation, treatment, and use of those names."
Zoological nomenclature is independent of other systems of nomenclature, for example botanical nomenclature. This implies that animals can have the same generic names as plants.
The rules and recommendations have one fundamental aim: to provide the maximum universality and continuity in the naming of all animals, except where taxonomic judgment dictates otherwise. The Code is meant to guide only the nomenclature of animals, while leaving zoologists freedom in classifying new taxa.
In other words, whether a species itself is or is not a recognized entity is a subjective decision, but what name should be applied to it is not. The Code applies only to the latter, not to the former. A new animal name published without adherence to the Code may be deemed simply "unavailable" if it fails to meet certain criteria, or fall entirely out of the province of science (e.g., the "scientific name" for the Loch Ness Monster).
The rules in the Code determine what names are valid for any taxon in the family group, genus group, and species group. It has additional (but more limited) provisions on names in higher ranks. The Code recognizes no case law. Any dispute is decided first by applying the Code directly, and not by reference to precedent.
The Code is also retroactive or retrospective, which means that previous editions of the Code, or previous other rules and conventions have no force any more today, and the nomenclatural acts published 'back in the old times' must be evaluated only under the present edition of the Code. In cases of disputes concerning the interpretation, the usual procedure is to consult the French Code, lastly a case can be brought to the Commission who has the right to publish a final decision.
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):
Principle of Binominal Nomenclature 
This is the principle that the scientific name of a species, and not of a taxon at any other rank, is a combination of two names; the use of a trinomen for the name of a subspecies and of uninominal names for taxa above the species group is in accord with this principle.
This means that in the system of nomenclature for animals the name of a species is composed of a combination of a generic name and a specific name, together they make a "binomen". No other rank can have a name composed of two names. Example:
- Species Giraffa camelopardalis
- Subspecies have a name composed of three names, a "trinomen": generic name, specific name, subspecific name:
- Subspecies Giraffa camelopardalis rothschildi
- Taxa at a rank above species have a name composed of one name, a "uninominal name".
In botanical nomenclature, the equivalent for "binominal nomenclature" is "binary nomenclature" (or sometimes "binomial nomenclature").
Principle of Priority 
This is the principle that the correct formal scientific name for an animal taxon, the valid name, correct to use, is the oldest available name that applies to it. It is the most important principle—the fundamental guiding precept that preserves zoological nomenclature stability. It was first formulated in 1842 by a committee appointed by the British Association to consider the rules of zoological nomenclature. Hugh Edwin Strickland wrote the committee's report.
- Nunneley 1837 established Limax maculatus (Gastropoda), Wiktor 2001 classified it as a junior synonym of Limax maximus Linnæus 1758 from S and W Europe. Limax maximus was established first, so if Wiktor's 2001 classification is accepted, Limax maximus takes precedence over Limax maculatus and must be used for the species.
There are approximately 2-3 million cases of this kind for which this Principle is applied in zoology.
Principle of Coordination 
The principle of coordination is that within the family group, genus group and species group, a name established for a taxon at any rank in the group is simultaneously established with the same author and date for taxa based on the same name-bearing type at other ranks in the corresponding group. In other words, publishing a new zoological name automatically and simultaneously establishes all corresponding names in the relevant other ranks with the same type.
In the species-group, publishing a species name (the binomen) Giraffa camelopardalis Linnaeus, 1758 also establishes the subspecies name (the trinomen) Giraffa camelopardalis camelopardalis Linnaeus, 1758. The same applies to the name of a subspecies; this establishes the corresponding species name.
In the genus-group, similarly, publishing the name of a genus also establishes the corresponding name of a subgenus (or vice versa): genus Giraffa Linnaeus, 1758 and subgenus Giraffa (Giraffa) Linnaeus, 1758.
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.
Principle of the First Reviser 
This is the principle that in cases of conflicts between simultaneously published divergent acts, the first subsequent author can decide which has precedence. It supplements the principle of priority, which states that the first published name takes precedence. The Principle of the First Reviser deals with situations that cannot be resolved by priority. These items may be two or more different names for the same taxon, two or more names with the same spelling used for different taxa, two or more different spellings of a particular name, etc. In such cases, the first subsequent author who deals with the matter chooses and publishes the decision in the required manner is the First Reviser, and is to be followed.
Linnæus 1758 established Strix scandiaca and Strix noctua (Aves), for which he gave different descriptions and referred to different types, but both taxa later turned out to refer to the same species, the snowy owl. The two names are subjective synonyms. Lönnberg 1931 acted as First Reviser, cited both names and selected Strix scandiaca to have precedence.
A problem is that sometimes the First Reviser is unknown. For the sperm whale Linnæus 1758 established three subjective synonyms, Physeter macrocephalus, Physeter catodon, and Physeter microps. The First Reviser remains unknown, currently both Ph. macrocephalus and Ph. catodon are used.
Principle of Homonymy 
This is the principle that the name of each taxon must be unique. Consequently a name that is a junior homonym of another name must not be used as a valid name.
It means that any one animal name, in one particular spelling, may be used only once (within its group). This is usually the first-published name; any later name with the same spelling (a homonym) is barred from being used. The Principles of Priority and the First Reviser apply here. For family-group names the termination (which is rank-bound) is not taken into account.
Genera are homonyms only if exactly the same — a one-letter difference is enough to distinguish them.
- Argus Bohadsch, 1761 (Gastropoda) (was made available for homonymy by ICZN in Opinion 429, Bohadsch 1761 was non-binominal - this had the effect that no other one of the various following names Argus can be used for a taxon)
- Argus Scopoli, 1763 (Lepidoptera: Lycaenidae: Polyommatinae)
- Argus Scopoli, 1777 (Lepidoptera: Nymphalidae: Satyrinae)
- Argus Poli, 1791 (Bivalvia)
- Argus Temminck, 1807 (Aves)
- Argus Lamarck, 1817 (Lepidoptera: Hesperiidae)
- Argus Walckenaer, 1836 (Araneae)
- Argus Gerhard, 1850 (Lepidoptera: Lycaenidae: Theclinae)
- Homonyms of Argus are not:
- Argua Walker, 1863 (Lepidoptera), Argusa Kelham, 1888 (Aves), Argusina Hebard, 1927 (Dermaptera), Arcus[disambiguation needed] Hong, 1983 (Diptera), Argas Latreille, 1795 (Araneae), Argulus Müller, 1785 (Crustacea).
- Homonyms are not: Isomya Cutler & Cutler, 1985 (Sipunculida), Isomyia Walker, 1859 (Diptera).
- Homonyms are not: Adelomya Mulsant & Verreaux, 1866 (Aves), Adelomyia Bonaparte, 1854 (Aves), Adelomys Gervais, 1853 (Mammalia), Adolomys Shevyreva, 1989 (Mammalia), Adulomya Kuroda, 1931 (Bivalvia).
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 Art. 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.
- Drury (1773) established Cerambyx maculatus (Coleoptera) for a species from Jamaica. Fueßlin (1775) established Cerambyx maculatus for a different species from Switzerland, and did not refer to Drury's name. Fueßlin's name is a junior primary homonym.
- Scopoli (1763) established Curculio fasciatus (Coleoptera) for a species from Slovenia. Strøm (1768) established Curculio fasciatus for another species from Norway. De Geer (1775) established Curculio fasciatus for a 3rd species from Sweden. Müller (1776) established Curculio fasciatus for a 4th species from Denmark. Fourcroy (1785) established Curculio fasciatus for a 5th species from France. Olivier (1790) established Curculio fasciatus for a 6th species from France. Marsham (1802) established Curculio fasciatus for a 7th species from Britain. All these names had descriptions that clarified that different species were meant, and that their authors did not know that the name had been established by a previous author.
Secondary homonyms can be produced if taxa with the same specific name but different original genus are later classified in the same genus (Art. 57.3, 59). A secondary synonym is only a temporary state, it is only effective in this classification. If another classification is applied, the secondary homonymy may not be produced, and the involved name can be used again (Art. 59.1). A name does not become unavailable or unusable if it was once in the course of history placed in such a genus where it produced a secondary homonymy with another name. This is one of the rare cases where a zoological species does not have a stable specific name and a unique species-author-year combination, it can have two names at the same time.
- Nunneley (1837) established Limax maculatus (Gastropoda), Wiktor (2001) classified it as a junior synonym of Limax (Limax) maximus Linnæus, 1758 from S and W Europe. Kaleniczenko, 1851 established Krynickillus maculatus for a different species from Ukraine. Wiktor, 2001 classified both Limax maximus Linnæus, 1758 and Krynickillus maculatus Kaleniczenko, 1851 in the genus Limax. This meant that L. maculatus Nunneley, 1837 and K. maculatus Kaleniczenko, 1851 were classified in the same genus, so both names were secondary homonyms in the genus Limax, and the younger name (from 1851) could not be used for the Ukrainian species. This made it necessary to look for the next younger available name that could be used for the Ukrainian species. This was Limax ecarinatus Boettger, 1881, a junior synonym of K. maculatus Kaleniczenko, 1851.
- For Wiktor (2001) and those authors who follow Wiktor's system the name of the Ukrainian species must be Limax ecarinatus Boettger, 1881. For the others who classify Limacus as a separate genus, the name of the Ukrainian species must be Limacus maculatus (Kaleniczenko, 1851).
- So the Ukrainian species can have two names, depending from its generic classification. Limax ecarinatus, Limacus maculatus, the same species.
Art. 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.
- Glischrus caelata Studer, 1820 (Gastropoda) was once classified in the genus Helix, and became a junior secondary homonym of Helix caelata [Vallot], 1801. Locard (1880) established a replacement name Helix glypta, which has very rarely been used. The species is currently known as Trochulus caelatus (Studer, 1820), and Art. 59.3 is commonly ignored.
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.
- The name Noctua Linnæus, 1758 was established for a lepidopteran subgenus. In 1764 he established a genus Noctua Linné ,1764 for birds, ignoring that he had already used this name a few years ago in Lepidoptera. Noctua Linné, 1764 (Aves) is a junior homonym of Noctua Linnæus, 1758 (Lepidoptera).
- Garsault (1764) used Noctua for a bird and established a name Noctua caprimulgus Garsault, 1764 (Aves). Fabricius (1775) used established a name Noctua caprimulgus Fabricius, 1775 (Lepidoptera), thus creating a double homonym. Double homonymy is no homonymy, both names are available.
- The same happened with Noctua variegata Jung, 1792 (Lepidoptera) and Noctua variegata Quoy & Gaimard, 1830 (Aves).
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.
- Echidna Forster, 1777 (Actinopterygii), not Echidna Cuvier, 1797 (Mammalia)
- Ansa Walker, 1858 (Lepidoptera), not Ansa Walker, 1868 (Hemiptera)
- Helix balcanica Kobelt, 1876, not Helix balcanica Kobelt, 1903 (both Gastropoda)
- Conus catenatus Sowerby, 1850, not Conus catenatus Sowerby, 1875 (both Gastropoda)
The name Ansa can only be used for a lepidopteran taxon. If that name cannot be used (for example because an older name established prior to 1858 takes precedence), this does not mean that the 1868 name can be used for a hemipteran genus. The only option to use the 1868 name for the hemipteran taxon is to get the 1858 name officially suppressed by the Commission.
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 would be useful to cite the page where the name was established.
- Amydona Walker, 1855 (Lepidoptera: Limacodidae) (p. 1110), not Amydona Walker, 1855 (Lepidoptera: Lasiocampidae) (p. 1413)
- Betousa Walker, 1865 (Lepidoptera: Thyridae) (p. 1111), not Betousa Walker, 1865 (Lepidoptera: Noctuidae) (p. 1208).
- Cicada variegata Fabricius, 1775 (p. 684), not Cicada variegata Fabricius, 1775 (p. 686) (both Auchenorrhyncha).
- Noctua marginata Fabricius, 1775 (p. 597), not Noctua marginata Fabricius, 1775 (p. 610) (both Lepidoptera: Noctuidae).
- Clausilia (Albinaria) oertzeni Boettger, 1889 (p. 42), not Clausilia (Albinaria) schuchi var. oertzeni Boettger, 1889 (p. 52) (both Gastropoda: Clausiliidae).
There are cases where two homonyms were established by the same author in the same year on the same page:
- Zonites verticillus var. graeca Kobelt, 1876 (Gastropoda) (p. 48), not Zonites albanicus var. graeca Kobelt, 1876 (p. 48).
Animal, plant, and fungi nomenclature are entirely independent from each other. The most evident shortcoming of this situation (for their use in biodiversity informatics) is that the same generic name can be used simultaneously for animals and plants. For this kind of homonyms the expression "hemihomonyms" is sometimes used. Far more than 1000 such names are known.
- The generic name Dryas L. (1753) represents a genus of magnoliophytan plants (family Rosaceae), and at the same time Dryas Hübner, 1807 is also a lepidopteran insect genus (family Nymphalidae).
- The genus Tandonia was established in animals (Gastropoda: Tandonia), in plants (Euphorbiaceae) and in Fungi (Ascomycetes).
- Other examples for sometimes well known plant names with zoological equivalents are Aotus (Fabaceae and Mammalia), Arenaria (Caryophyllaeceae and Aves), Betula(Betulaceae and Hymenoptera), Chloris (Cactaceae and Aves), Dugesia (Asteraceae and Plathelminthes), Erica (Ericaceae and Araneae), Hystrix (Poaceae and Mammalia), Iris (Asparagales and Orthoptera), Liparis (Orchidaceae and Actinopterygii), Phalaenopsis (Asparagales and Aves), Pinus (Pinaceae and Mollusca), Prunella (Lamiaceae and Aves), Ricinus (Fabaceae and Acari), Taxus (Taxaceae and Mammalia), Typha (Typhaceae and Porifera), Ulva (Ulvophyceae and Lepidoptera), Viola (Violaceae and Lepidoptera).
For names above the family level, the Principle of Homonymy does not apply.
- Pulmonata is usually used for a very prominent group in Gastropoda, but the name is also (rarely) used for a group in Arachnida.
- Reticulata is used as an order in Foraminifera, and as an undefined higher group in Ephemeroptera.
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.
- Bulimina (Foraminifera) and Buliminus (Gastropoda) give both Buliminidae, and both families were used since the 1880s. When the homonymy was discovered 110 years later in the 1990s, the younger (gastropod) taxon had to receive a new family name, and the Commission needed was asked for a solution (Opinion 2018).
- Claria (Rotifera) and Clarias (Actinopterygii) give both Clariidae, but only the actinopterygian fish name was used since 1845. Shortly after Clariidae had been proposed in Rotifera in 1990, the homonymy was discovered and the Commission had to decide that the Rotiferan family had to be amended to Clariaidae (Opinion 2032).
Principle of Typification 
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.
This means that any named taxon has a name-bearing type, which allows the objective application of that name. Any family-group name must have a type genus, any genus-group name must have a type species, and any species-group name can (not must) have one or more type specimens (holotype, lectotype, neotype, syntypes, or others), usually deposited in a museum collection. The type genus for a family-group name is simply the genus that provided the stem to which was added the ending -idae (for families). Example:
The type species for a genus-group name is more complicated and follows exactly defined provisions in Art. 67-69. Type species are very important, and no general zoological database has recorded the type species for all genera. Except in fishes and some minor groups, type species are rarely reliably recorded in online animal databases. In 60% of the cases the type species can be determined in the original publication. The type species is always the original name of the taxon (and not the currently used combination).
- The correctly cited type species of Locusta Linnæus, 1758 (Caelifera) is Gryllus migratorius Linnæus, 1758, not Locusta migratoria (Linnæus, 1758).
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 (Art. 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.
There are various possible modes of type species designation. This is their order of legal importance, with approximate proportions of occurrence and examples:
- Superior type fixation:
- Designation by ICZN under the plenary powers (3 %)
- Galba Schrank, 1803 (Gastropoda) was established with one species included, Galba pusilla Schrank, 1803. This would be the type species by monotypy. In Opinion 1896 (published in 1998) this type fixation was set aside and Buccinum truncatulum Müller, 1774 was fixed as type species under the plenary power(s) (currently Galba truncatula).
- Designation under Art. 70.3 (misidentified type species) (1 %)
- Bollingeria Forcart, 1940 (Gastropoda) was established with its type species Chondrus pupoides Krynicki, 1833 proposed by original designation. But Forcart 1940 misidentified the type species and meant Bulimus lamelliferus Rossmässler, 1858. It would be convenient to designate Bulimus lamelliferus as type species under Art. 70.3.
- Helisoma Swainson, 1840 (Gastropoda) was established with one species included, cited by Swainson as "H. bicarinata Sow. Gen. f. 4". This suggested that the type species was misidentified, and that Planorbis campanulatus Say, 1821 and not Planorbis bicarinatus Say, 1819 was meant. But since the incorrect type species Planorbis bicarinatus has been regarded as type, it would be convenient to fix this as type under Art. 70.3.
- Designation by ICZN under the plenary powers (3 %)
- Type fixation in the original work:
- Original designation (31 %)
- Montfort 1810 established the genus Theodoxus (Gastropoda) and designated Theodoxus lutetianus Montfort 1810 as type species (currently Theodoxus fluviatilis).
- Vest 1867 established the subgenus Clausilia (Isabellaria) (Gastropoda) and designated Clausilia isabellina Pfeiffer, 1842 as type species (currently Isabellaria isabellina).
- Riedel 1987 established the genus Turcozonites (Gastropoda) and designated Zonites wandae Riedel, 1982 as type species (currently Turcozonites wandae).
- Monotypy (28 %)
- Anodonta Lamarck, 1799 (Bivalvia) was originally established with one included nominal species, Mytilus cygneus Linnæus, 1758. This is the type species fixed by monotypy (currently Anodonta cygnea).
- Microcondylaea Vest 1866 (Bivalvia) was originally established with two included nominal species, Unio bonellii Férussac, 1827 and with doubts Anodonta lata Rafinesque, 1820. Doubtfully included species do not count, type species is Unio bonellii fixed by monotypy (currently Microcondylaea bonellii).
- Absolute tautonymy (2 %)
- Kobelt 1871 established the gastropod genus-group name Candidula and included 23 species. Among these was Glischrus candidula Studer 1820. Glischrus candidula is type species fixed by absolute tautonymy (currently Candidula unifasciata).
- Draparnaud 1801 established the gastropd genus Succinea and included two species, Succinea amphibia Draparnaud 1801 and Succinea oblonga Draparnaud 1801. Among the synonyms of S. amphibia, Draparnaud listed a name Helix succinea Müller 1774. Synonyms do count here, so Helix succinea is type species by absolute tautonymy (currently Succinea putris).
- Kobelt 1904 established the gastropod subgenus Iberus (Balearica) and included 10 species. Among these was Helix balearica Rossmässler 1838, which Kobelt cited as Iberus (Balearica) balearicus. The ending -us is irrelevant here, Helix balearica is type species by absolute tautonymy (currently Iberellus balearicus or Iberellus hispanicus).
- Euxinolauria Lindholm, 1924 (Gastropoda: Lauriidae) was established as a new replacement name for Caucasica Caziot & Margier, 1909 (not Caucasica Boettger, 1877 (Gastropoda: Clausiliidae)). Caucasica Caziot & Margier, 1909 contained originally 4 species, among which was Pupa caucasica Pfeiffer, 1857. This is the type species for Caucasica Caziot & Margier, 1909 fixed by absolute tautonymy, and also for Euxinolauria (currently Euxinolauria caucasica).
- The following examples do not represent absolute tautonymy: Scomber scombrus Linnæus, 1758 (Actinopterygii), Babyrousa babyrussa (Linnæus, 1758) (Mammalia), Suricata suricatta (Schreber, 1776) (Mammalia), Merlangius merlangus (Linnæus, 1758) (Actinopterygii), Isabellaria isabellina (Pfeiffer, 1842) (Gastropoda), Rupestrella rupestris (Philippi, 1836) (Gastropoda).
- Linnean tautonymy (0.3 %)
- Linnæus 1758 established Castor (Mammalia) and included two species, Castor fiber and Castor moschatus. Among the synonyms of Castor fiber was cited the one-word name Castor with references to six pre-Linnean works (Gesner 1598, Rondelet 1554, Jonston 1650, Dodart 1676, Ray 1693 and Aldrovandi 1649). Castor fiber Linnæus 1758 is type species fixed by Linnean tautonymy (currently Castor fiber).
- Original designation (31 %)
- Subsequent methods of type fixation:
- Subsequent monotypy (2 %)
- Valvata Müller, 1773 (Gastropoda) was established with a short description and without species. Müller 1774 included one species Valvata cristata Müller 1774. Valvata cristata is type species by subsequent monotypy (currently Valvata cristata).
- Omphiscola Rafinesque, 1819 (Gastropoda) was established without species included. Beck 1837  included one species Buccinum glabrum Müller, 1774. Buccinum glabrum is type species by subsequent monotypy (currently Omphiscola glabra).
- Subsequent absolute tautonymy (only very few cases)
- Alosa Garsault, 1764 (Actinopterygii) was established without included species. As first author, Cuvier, 1829 included two species Clupea alosa and Clupea fincta. Type species is Clupea alosa Linnæus 1758 by subsequent absolute tautonymy (currently Alosa alosa).
- Rupicapra Garsault, 1764 (Mammalia) was established without included species. As first author, Blainville, 1816 included 3 species Capra rupicapra Linnæus, 1758, Capra pudu, and Capra americana. Type species is Capra rupicapra by subsequent absolute tautonymy (currently Rupicapra rupicapra).
- Subsequent Linnean tautonymy (only theoretical, there might be no case)
- Subsequent designation (32 %)
- Aplexa Fleming, 1820 (Gastropoda) was established with two species, Bulla hypnorum Linnæus, 1758 and Bulla rivalis Turton, 1807. Herrmannsen 1846 fixed Bulla hypnorum as type by subsequent designation (currently Aplexa hypnorum).
- Pseudanodonta Bourguignat 1877 (Bivalvia) was established with 7 species, Anodonta complanata Rossmässler 1835, and 6 others. Westerlund 1902 validly designated Anodonta complanata as type species (currently Pseudanodonta complanata).
- Subsequent monotypy (2 %)
A species-group name can have a name-bearing type specimen, but this is no must. 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 (Art. 74.7.3, 75.2, 75.3).
- Aptenodytes patagonica Miller, 1778 is either based on a type specimen, perhaps deposited in the Natural History Museum London or somewhere else, or its type is lost. This is currently irrelevant because the usage of the name (as Aptenodytes patagonicus) for the king penguin is unambiguously accepted.
- The name-bearing type for Homo sapiens Linnæus, 1758 is deposited in Uppsala (the bones of Carl von Linné). This is a lectotype designated by Stearn 1959, correctly but unnecessarily because the usage of the name was unambiguous at that time, and still is.
Links to the separate articles (these are redundant and can eventually be removed):
- Principle of Coordination
- Principle of the First Reviser
- Principle of Homonymy
- Principle of Priority
- Principle of Typification
The Code divides names in the following manner:
- Names above the family group
- Family-group names
- Genus-group names
- Species-group names
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 names in the family, genus, and species groups are fully regulated by the provisions in the Code. There is no limitation to the number of ranks allowed in the family group. The genus group has only two ranks: genus and subgenus. The species group has only two ranks: species and subspecies.
Gender agreement 
In the species group gender agreement applies. The name of a species, in two parts, a binomen, say, Loxodonta africana, and of a subspecies, in three parts, a trinomen, say Canis lupus albus, is in the form of a Latin phrase, and must be grammatically correct Latin. If the second part, the specific name (or the third part, the subspecific name) is adjectival in nature, its ending must agree in gender with the name of the genus. If it is a noun, or an arbitrary combination of letters, this does not apply.
- For instance, the generic name Equus is masculine; in the name Equus africanus the specific name africanus is an adjective, and its ending follows the gender of the generic name.
- In Equus zebra the specific name zebra is a noun, it may not be "corrected" to "Equus zebrus".
- In Equus quagga burchellii the subspecific name burchellii is a noun in the genitive case ("of Burchell").
If a species is moved, therefore, the spelling of an ending may need to change. If Gryllus migratorius is moved to the genus Locusta, it becomes Locusta migratoria. Confusion over Latin grammar has led to many incorrectly formed names appearing in print. An automated search may fail to find all the variant spellings of a given name (e.g., the spellings atra and ater may refer to the same species).
Many laymen[who?], and some scientists[who?], object to continued adherence to this rule, especially those who work with butterflies and moths. This is for historical reasons. In 1758, Linnæus placed all butterflies in the genus Papilio, which, after a few decades, contained thousands of species. From the beginning, the gender of Papilio was unclear, undecided, and disputed. Some authors regarded it as masculine, others as feminine. Linnæus knew this problem and avoided any statement. All his 250 specific names in the genus Papilio were either nouns, indeclinable adjectives, or adjectives ending in -is (which can be masculine or feminine but not neuter). He did not use a single adjective ending in -us, -a, -um. P. Brown, Cramer, Fabricius, Fueßlin, Goeze, Poda and Schrank regarded Papilio as masculine, Ménétriés, Pontoppidan and most modern authors as feminine. In ICZN Opinion 278 from 1954, it was regarded as masculine. In many cases lepidopterists would not change the ending of a name as used by the author who established a name. So we find for example Papilio fuscus or Papilio macilentus, but also Papilio osmana and Papilio paradoxa. Only in a few cases are both versions found in the Web (an example is Papilio multicaudatus and Papilio multicaudata). This works also with other butterfly genera of which the gender is undisputed. Graphium appears neuter, but only the inconsistent versions Graphium angolanus and Graphium mandarinus are used, while Graphium sandawanum can only be found with a neuter specific name. Likewise, pairs are more frequently found in genera of which the gender is not obvious: Delias castaneus and Delias gigantea, Belenois albumaculatus and Belenois rubrosignata, Mylothris arabicus and Mylothris ruandana. Even in moths, such pairs occur: Xylophanes obscurus and Xylophanes turbata, Manduca boliviana and Manduca caribbeus, Sphinx caligineus and Sphinx formosana, Macroglossum albolineata and Macroglossum vicinum. It may also occur that a lepidopteran subspecies can have a different gender from the name of the species, as for example in Papilio multicaudata pusillus Austin & Emmel, 1998, and Papilio torquatus flavida Oberthür, 1879.
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 International Commission on Zoological Nomenclature, acting on behalf of all zoologists. The Commission takes such action in response to proposals submitted to it.
- Carolus Linnaeus named the Domestic Cat Felis catus in 1758; Johann Christian Daniel von Schreber named the Wildcat Felis silvestris in 1775. For taxonomists who consider these two kinds of cat a single species the Principle of Priority means that the species ought to be named F. catus, but in practice almost all biologists have used F. silvestris. In 2003, the Commission issued a ruling (Opinion 2027) that "conserved the usage of 17 specific names based on wild species, which are pre-dated, by or contemporary with those based on domestic forms", confirming F. silvestris for the wild cat. Taxonomists who consider the domesticated cat the same species as the wild cat should use F. silvestris; taxonomists who consider the domesticated cat a subspecies of the wild cat should use F. silvestris catus; taxonomists who consider the domesticated cat a separate species should use F. catus.
The latest amendments enacted by the Commission concern electronic publishing, which is now permitted for works published under an ISBN or ISSN after 2011 in a way that ensures registration with ZooBank as well as archival of multiple copies.
Local usage and name changes 
The ICZN is used by the scientific community worldwide. Changes are governed by guidelines in the code. Local changes, such as the changes proposed by the Turkish government, are not recognised by ICZN.
See also 
- International Code of Nomenclature for algae, fungi, and plants
- Nomen dubium
- Nomen nudum
- Nomen oblitum
- Allen, JA (1897). "The Merton Rules". Science 6 (131): 9–19. doi:10.1126/science.6.131.9. PMID 17819182.
- Strickland, HE (1878). Rules for Zoological Nomenclature. John Murray, London.
- Dayrat, B (2010). "Celebrating 250 Dynamic Years of Nomenclatural Debates". In Polaszek, A. Systema Naturae 250 - The Linnaean Ark. Taylor and Francis.
- Blanchard, R., Maehrenthal, F. von & Stiles, C. W. 1905. Règles internationales de la nomenclature zoologique adoptées par les Congrès International de Zoologie. International Rules of Zoological Nomenclature. Internationale Regeln der Zoologischen Nomenklatur. - Paris (Rudeval)
- pp. V-VI in Kraus, O. 1962. Internationale Regeln für die Zoologische Nomenklatur. Beschlossen vom XV. Internationalen Kongress für Zoologie. - pp. I-VIII [= 1-8], 1-90. Frankfurt am Main. (Senckenbergische Naturforschende Gesellschaft).
- ICZN Code Art. 77.2
- ICZN Constitution Art. 16.2
- French Code online
- ICZN Code Art. 86.2
- ICZN Code Art. 86.3
- ICZN Code Art. 89
- ICZN Code Glossary
- ICZN Code Glossary, "binomen"
- ICZN Code Art. 24.2.
- The publication by [Vallot] (1801) has not been unambiguously recognized as published work in the sense of the Code Art. 8, which might be another reason to ignore Art. 59.3 in this case.
- Moscow State University hemihomonyms database
- These proportions apply to 366 verified European non-marine mollusc genera ([www.animalbase.org]), presumed to represent a more-or-less representative animal group.
- "Subsequent absolute tautonymy" is not used as a term in the Code's 4th edition, but it is a logical consequence of the usage of the term "subsequent monotypy".
- AnimalBase: List of all taxa, sorted by genus names, Papilio
- "Opinion 2027 (Case 3010): Usage of 17 specific names based on wild species that are pre-dated by, or contemporary with, those based on domestic animals (Lepidoptera, Osteichthyes, Mammalia)". Bulletin of Zoological Nomenclature (International Commission on Zoological Nomenclature) 60 (1). 31 March 2003. Retrieved 8 October 2008.
- International Commission on Zoological Nomenclature (2012). "Amendment of Articles 8, 9, 10, 21 and 78 of the International Code of Zoological Nomenclature to expand and refine methods of publication". ZooKeys 219: 1–0. doi:10.3897/zookeys.219.3944.
- Scott L. Wing Causes and Consequences of Globally Warm Climates in the Early ... - 2003 No 369 - Page 288 "Following the general practice of naming species after localities by ending with "-ensis," Schnack (2000) proposed to change the name Discorbis duwi to Discorbis duwiensis. However, the International Code of Zoological Nomenclature (Chapter 7 Article 32) does not allow such a change"