International Commission on Zoological Nomenclature
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Article 24. Precedence between simultaneously published names, spellings or acts.

24.1. Automatic determination of precedence of names. When homonyms or synonyms are established simultaneously, but proposed at different ranks, in the family group, genus group or species group the name proposed at higher rank takes precedence [Arts. 55.5, 56.3, 57.7]. See Article 61.2.1 for the precedence of simultaneous but different type fixations for taxa and their nominotypical subordinate taxa.

Example. The simultaneously established species-group names vulgaris Schmidt and sinensis Chang are considered to be synonyms; sinensis, proposed for a species, takes precedence over vulgaris because the latter was proposed for a subspecies.

24.2. Determination by the First Reviser.

24.2.1. Statement of the Principle of the First Reviser. When the precedence between names or nomenclatural acts cannot be objectively determined, the precedence is fixed by the action of the first author citing in a published work those names or acts and selecting from them; this author is termed the "First Reviser".

24.2.2. Determination of precedence of names or acts by the First Reviser. If two or more names, different or identical, and based on the same or different types, or two or more nomenclatural acts, are published on the same date in the same or different works, the precedence of the names or acts is fixed by the First Reviser unless Article 24.1 applies.

Example. The names Strix scandiaca and S. nyctea (Aves) were published together by Linnaeus (1758) and are considered to be subjective synonyms. Lönnberg (1931) acted as First Reviser and gave precedence to the name Strix scandiaca; thus, the valid name for the species (the Snowy Owl) is Nyctea scandiaca (Linnaeus, 1758) rather than N. nyctea (Linnaeus, 1758).

24.2.3. Selection of correct original spellings. If a name is spelled in more than one way in the original work, the first author to have cited them together and to have selected one spelling as correct is the First Reviser. The selected spelling (if not incorrect under Articles 32.4. or 32.5) is thereby fixed as the correct original spelling; any other spelling is incorrect (and therefore unavailable [Art. 32.4]).

24.2.4. Original authors may be deemed to be First Revisers of spellings. When the author, or one of joint authors, of two different original spellings of the same name subsequently uses one of them as valid in a work (including the author's or publisher's corrigenda), and neither had previously been selected as the correct spelling by a First Reviser, the author is deemed to be the First Reviser, whether or not the author cites both spellings together (that used as valid becomes the correct original spelling).

24.2.5. Unnecessary action by a First Reviser. If it is shown subsequently that the precedence of names, spellings or acts can be objectively determined, the action of the First Reviser is nullified.

Recommendation 24A. Action of First Reviser. In acting as First Reviser in the meaning of this Article, an author should select the name, spelling or nomenclatural act that will best serve stability and universality of nomenclature.

Recommendation 24B. First Revisers choosing between identical names should follow contemporary attributions of authorship. Zoologists acting as First Revisers to determine the precedence of identical names published in the same or different works, and on the same day, are advised to follow attributions by the authors concerned if these are known (see Article 50.6).