Synonyms are names that are spelled differently but refer to the same taxon. In this code, synonyms must be established clade names and may be homodefinitional (based on the same definition) or heterodefinitional (based on different definitions). The criteria for determining whether definitions are different are described in Article 13.2, including Note 13.2.1.
Homodefinitional synonyms are synonyms regardless of the phylogenetic context in which the names are applied. However, in the case of names with different definitions, the phylogenetic context determines whether the names are heterodefinitional synonyms or not synonymous.
Suppose that Hypothetica were defined as the smallest clade containing species A and B, and Cladia were defined as the smallest clade containing species C and B. In the context of any hypothesized phylogeny in which A shares a more recent common ancestor with C than either does with B, Hypothetica and Cladia would be heterodefinitional synonyms. However, in the context of an alternative hypothesis that A and B are more closely related to each other than either is to C, Hypothetica and Cladia would not be synonymous.
Minimum-clade, apomorphy-based, and maximum-clade definitions (Arts. 9.5–9.7) usually designate different clades, although they may be nested clades that differ only slightly in inclusiveness. Therefore, names based on two or more of these different kinds of definitions usually are not synonyms. (In theory, it is possible for different types of definitions to designate the same clade. For example, in cases in which doubling of the chromosomes (autopolyploidy) causes speciation, the apomorphic chromosome number arises simultaneously with the splitting of a lineage. In such cases, an apomorphy-based definition that uses this chromosome number as a specifier will refer to the same clade as a maximum-clade definition that uses the species in which the chromosome doubling occurred, or one of its descendants, as the internal specifier if an appropriate external specifier is used.)
If there are two or more synonyms for a clade, the accepted name for that clade is the earliest acceptable one that applies to it, except in cases of conservation (Art. 15), or precedence of a panclade name (Art. 14.4), or precedence of a younger panclade name over an older one to maintain consistency with crown-clade names (Art. 14.5).
When two or more synonyms have the same publication date (Art. 5), the one that was registered first (and therefore has the lowest registration number) takes precedence.
If a panclade name (Art. 10.3) and a name that was not explicitly established as applying to a total clade are judged to be heterodefinitional synonyms (Art. 14.1), the panclade name has precedence even if it was established later (except in cases covered by Art. 10.7).
In order to maintain the relationships between panclade names and the crown-clade names upon which they are based, precedence among panclade names is based on precedence of the corresponding crown-clade names. Thus, if two or more panclade names are considered synonyms because the names of the crown clade upon which they are based are considered synonyms, the panclade name that has precedence is the one that is based on the crown-clade name that has precedence, and that is the case even if one or more of the other panclade names were established earlier than the one based on the crown-clade name that has precedence.
Suppose that first the crown-clade name Reptilia was established, then later the crown-clade name Lepidosauria and the panclade name Pan-Lepidosauria were established simultaneously, then later still the panclade name Pan-Reptilia was established. In the context of phylogenies in which Reptilia and Lepidosauria are synonyms, Reptilia would be the accepted name of the crown clade, and Pan-Reptilia would be the accepted name of the corresponding total clade even though Pan-Lepidosauria was established before Pan-Reptilia.