Article 23. Principle of Priority.

23.1. Statement of the Principle of Priority. The valid name of a taxon is the oldest available name applied to it, unless that name has been invalidated or another name is given precedence by any provision of the Code or by any ruling of the Commission. For this reason priority applies to the validity of synonyms [Art. 23.3], to the relative precedence of homonyms [Arts. 53-60], the correctness or otherwise of spellings [Arts. 24, 32], and to the validity of nomenclatural acts (such as acts taken under the Principle of the First Reviser [Art. 24.2] and the fixation of name-bearing types [Arts. 68, 69, 74.1.3, 75.4]).

23.1.1. For exceptions for certain family-group names see Articles 35.5 and 40.

23.1.2. For the case of disused family-group names which are homonyms see Article 55.3.1.1.

23.1.3. For the circumstances in which certain genus-group names are excluded from application of the Principle of Priority see Articles 20 and 23.7.

23.1.4. For the circumstances in which certain species-group names are partly excluded from the application of the Principle of Priority see Articles 23.7.3 and 23.8.

23.2. Purpose. In accordance with the objects of the Code (see Preamble), the Principle of Priority is to be used to promote stability and it is not intended to be used to upset a long-accepted name in its accustomed meaning by the introduction of a name that is its senior synonym or homonym (for certain such cases see Article 23.9), or through an action taken following the discovery of a prior and hitherto unrecognized nomenclatural act (such as a prior type fixation; for such cases see Articles 70.2 and 75.6).

23.3. Application to Synonymy. The Principle of Priority requires that a taxon formed by bringing together into a single taxon at one rank two or more previously established nominal taxa within the family group, genus group or species group takes as its valid name the name determined in accordance with the Principle of Priority [Art. 23.1] and its Purpose [Art. 23.2], with change of suffix if required in the case of a family-group name [Art. 34].

Example. The valid name of a genus formed by the union of the genera Aus 1850 and Cus 1870, and the subgenus Bus 1800 (transferred from the genus Xus 1758), is Bus 1800.

23.3.1. Priority of the name of a nominal taxon is not affected by elevation or reduction in rank of the taxon within the family group, genus group or species group [Arts. 36, 43, 46], nor by any mandatory change in suffix of a family-group name consequent upon change in rank [Art. 34].

23.3.2. The Principle of Priority applies even if

23.3.2.1. any part of an animal is named before the whole animal, or

23.3.2.2. two or more generations, forms, stages, or sexes of a species are named as different nominal taxa, or

23.3.2.3. a name was established before 1931 on the work of an extant animal before one was established for the animal itself (for ichnotaxa see Article 23.7).

23.3.3. The Principle of Priority applies to interpolated specific names added in parenthesis after a genus-group name to denote aggregates of species or interpolated in parenthesis between specific and subspecific names to denote aggregates of subspecies [Art. 6.2]. The precedence of such an interpolated name is that which it has in the species group (see Article 11.9.3.5).

23.3.4. The Principle of Priority does not apply to names applied to infrasubspecific entities, since they are excluded from zoological nomenclature [Art. 1.3.4]. If a name which had been published for such an entity is later established for a species or subspecies (see Articles 10.2, 45.5 and 45.6), then the Principle of Priority applies from the date the name becomes available as the result of that establishment.

23.3.5. The Principle of Priority requires that if a name in use for a taxon is found to be unavailable or invalid it must be replaced by the next oldest available name from among its synonyms, including the names of the contained taxa of the same group (e.g. subgenera within genera), providing that that name is not itself invalid. If the rejected name has no potentially valid synonym a new substitute name (see Article 60.3) must be established in its place.

Examples. The genus Aus 1850 is considered to contain subgenera with the valid names Aus 1850, Bus 1900 and Cus 1860. If the name Aus is found to be unavailable or invalid, the name of the genus and nominotypical subgenus becomes Cus 1860; however, if the former subgenus Aus (Aus) had a synonym Dus 1855 (i.e. it contains the type species of Dus) then the name of the genus becomes Dus 1855.

23.3.6. The Principle of Priority continues to apply to an available name when treated as a junior synonym; it may be used as the valid name of a taxon by an author who considers the synonymy to be erroneous, or if the senior synonym is found to be unavailable or invalid (for names first published as junior synonyms, see Article 11.6).

23.3.7. An available name valid according to the Principle of Priority is not to be rejected, even by its author(s), for a reason such as its inappropriateness or tautonymy (for examples see Article 18), or incorrect spelling (such a name remains valid, but in its correct form: see Article 19).

23.4. Application to Homonymy. The Principle of Priority requires that the relative precedence of homonyms, including secondary homonyms in the species group, is determined in accordance with the Principle of Priority (see Articles 23.1 and 23.2) and the Principle of Homonymy [Art. 52]; for its application to homonyms published simultaneously, see Article 24.

23.4.1. The Principle of Priority applies to a family-group name if either the name itself or the name of its type genus is found to be a junior homonym; for such cases see Articles 55 and 39 respectively.

23.5. Application to spellings. The Principle of Priority applies to the spellings of an available name, unless an incorrect spelling has been preserved in accordance with Article 33.3.1, or, in the case of family-group names, with Articles 29.4 or 29.5. (For the preservation of unjustified emendations see Article 33.2.3.1).

23.6. Application to nomenclatural acts. In accordance with the Principle of Priority the first nomenclatural act taken in respect of a name or a nominal taxon to achieve any of the following constitutes the only valid such act: i.e. acts taken under the First Reviser Principle [Art. 24.2], fixation of type species [Arts. 68, 69], first inclusion of nominal species in a genus-group taxon [Art. 67.2], designation of lectotypes [Art. 74.1.3] and neotypes [Art. 75.5] (types in the family group are fixed automatically and are not subject to subsequent fixation [Art. 63]; but for names published after 1999 see Article 16.2).

23.7. Application to collective groups and ichnotaxa. Except for the application of the Principle of Homonymy [Arts. 55, 56, 57],

23.7.1. a name established expressly for a collective group does not compete in priority with other genus-group names;

23.7.2. a name established for a nominal genus-group taxon but subsequently brought into use for a collective group no longer competes in priority with other genus-group names while so used (see also Article 67.14);

23.7.3. a name established for an ichnotaxon does not compete in priority with a name established for an animal (even for the animal that formed, or may have formed, the trace fossil).

Example. Krebs (1966) associated the footprints named Chirotherium by Kaup (1835) with the Triassic fossil reptile Ticinosuchus Krebs, 1965. Ticinosuchus must not be rejected as a junior synonym of Chirotherium on that account.

23.8. Application to species-group names established on hybrids. A species-group name established for an animal later found to be a hybrid [Art. 17] must not be used as the valid name for either of the parental species, even if it is older than all other available names for them. Such a name may enter into homonymy. For names based on taxa which are of hybrid origin see Article 17.2.

23.9. Reversal of precedence. In accordance with the purpose of the Principle of Priority [Art. 23.2], its application is moderated as follows:

23.9.1. prevailing usage must be maintained when the following conditions are both met:

23.9.1.1. the senior synonym or homonym has not been used as a valid name after 1899, and

23.9.1.2. the junior synonym or homonym has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years and encompassing a span of not less than 10 years.

23.9.2. An author who discovers that both the conditions of 23.9.1 are met should cite the two names together and state explicitly that the younger name is valid, and that the action is taken in accordance with this Article; at the same time the author must give evidence that the conditions of Article 23.9.1.2 are met, and also state that, to his or her knowledge, the condition in Article 23.9.1.1 applies. From the date of publication of that act the younger name has precedence over the older name. When cited, the younger but valid name may be qualified by the term nomen protectum and the invalid, but older, name by the term nomen oblitum (see Glossary). In the case of subjective synonymy, whenever the names are not regarded as synonyms the older name may be used as valid.

Example. The valid name of a species formed by including the nominal taxa Aus xus Schmidt, 1940 and Aus wus Jones, 1800 in a single taxonomic species is Aus wus Jones, 1800. But if the conditions in Article 23.9.1.1 and 23.9.1.2 are met, then Aus xus Schmidt, 1940 becomes (unless the Commission rules otherwise) the valid name of that species. However, if the nominal taxa do refer to separate taxonomic species the names of these are Aus xus Schmidt, 1940 and Aus wus Jones, 1800. If, on the other hand, the two taxa are treated as subspecies of a single species then the names of these are Aus xus xus Schmidt, 1940 and Aus xus wus Jones, 1800 - not Aus wus xus Schmidt, 1940 and Aus wus wus Jones, 1800.

Recommendation 23A. If suppression desired. If in the opinion of an author suppression of the older name, rather than a change in the relative precedence of the two names involved, is desirable, in addition to taking action under Article 23.9.2 to maintain prevailing usage, the author should refer the case to the Commission with an appropriate recommendation for a ruling.

23.9.3. If the conditions of 23.9.1 are not met but nevertheless an author considers that the use of the older synonym or homonym would threaten stability or universality or cause confusion, and so wishes to maintain use of the younger synonym or homonym, he or she must refer the matter to the Commission for a ruling under the plenary power [Art. 81]. While the case is under consideration use of the junior name is to be maintained [Art. 82].

23.9.4. If the case is one of homonymy in family-group names resulting from similarity but not identity in the names of type-genera, see Article 55.3.

23.9.5. When an author discovers that a species-group name in use is a junior primary homonym [Art. 53.3] of another species-group name also in use, but the names apply to taxa not considered congeneric after 1899, the author must not automatically replace the junior homonym; the case should be referred to the Commission for a ruling under the plenary power and meanwhile prevailing usage of both names is to be maintained [Art. 82].

23.9.6. The deliberate use of a name contrary to Article 23.9.1, or the mentioning of a name in a synonymy, or its mere listing in an abstracting publication, or in a nomenclator or other index or list of names must not be taken into account in determining usage under Articles 23.9.1.1 and 23.9.1.2.

23.10. Erroneous reversal of precedence. If action taken under Article 23.9.2 is found later to have been taken in error in that conditions 23.9.1.1 and 23.9.1.2 were not met, the case is to be referred to the Commission. Prevailing usage must be maintained [Art. 82] until the Commission has made a ruling (i.e. an author discovering that such an erroneous action has occurred must not automatically use the older synonym or homonym).

23.11. Application of strict priority desired. If an author wishes to replace a name in prevailing usage by its older synonym when the conditions of Article 23.9.1 are met, he or she must apply to the Commission for a ruling under the plenary power [Art. 81].

23.12. Names rejected under former Article 23b. A name that was rejected between 6 November 1961 and 1 January 1973, by an author who explicitly applied Article 23b in force between those dates under the then current editions of the Code, on the grounds that it was a nomen oblitum (see Glossary) is not to be given precedence over a junior synonym in prevailing usage, unless the Commission rules that the older but rejected name is to take precedence.

23.12.1. The term "rejected" in this Article must be construed rigidly; mere disregarding of a name is not to be construed as rejection (even if the Article 23b, then in force, was mentioned). The rejected name must have been cited and a junior synonym used instead of it as the valid name.

23.12.2. A name which was rejected under the former Article 23b may, in the absence of any other cause of invalidity, be used as valid if it is no longer considered to be a synonym of another name, or if its synonyms are themselves invalid under the provisions of the Code.