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11.1. Each family or taxon of lower rank with a particular circumscription, position, and rank can bear only one correct name, special exceptions being made for 9 families and 1 subfamily for which alternative names are permitted (see Art. 18.5 and 19.7). However, the use of separate names for the form-taxa of fungi and for morphotaxa of fossil plants is allowed under Art. 1.3, 59.4, and 59.5.
11.2. In no case does a name have priority outside the rank in which it is published (but see Art. 53.4).
Ex. 1. Campanula sect. Campanopsis R. Br. (Prodr.: 561. 1810) when treated as a genus is called Wahlenbergia Roth (1821), a name conserved against the taxonomic (heterotypic) synonym Cervicina Delile (1813), and not Campanopsis (R. Br.) Kuntze (1891).
Ex. 2. Magnolia virginiana var. foetida L. (1753) when raised to specific rank is called M. grandiflora L. (1759), not M. foetida (L.) Sarg. (1889).
Ex. 3. Lythrum intermedium Ledeb. (1822) when treated as a variety of L. salicaria L. (1753) is called L. salicaria var. glabrum Ledeb. (Fl. Ross. 2: 127. 1843), not L. salicaria var. intermedium (Ledeb.) Koehne (in Bot. Jahrb. Syst. 1: 327. 1881).
Ex. 4. When the two varieties constituting Hemerocallis lilioasphodelus L. (1753), var. flava L. and var. fulva L., are considered to be distinct species, the one not including the lectotype of the species name is called H. fulva (L.) L. (1762), but the other one bears the name H. lilioasphodelus L., which in the rank of species has priority over H. flava (L.) L. (1762).
11.3. For any taxon from family to genus inclusive, the correct name is the earliest legitimate one with the same rank, except in cases of limitation of priority by conservation (see Art. 14) or where Art. 11.7, 15, 19.4, 56, 57, or 59 apply.
Ex. 5. When Aesculus L. (1753), Pavia Mill. (1754), Macrothyrsus Spach (1834) and Calothyrsus Spach (1834) are referred to a single genus, its name is Aesculus L.
11.4. For any taxon below the rank of genus, the correct name is the combination of the final epithet of the earliest legitimate name of the taxon in the same rank, with the correct name of the genus or species to which it is assigned, except (a) in cases of limitation of priority under Art. 14, 15, 56, or 57, or (b) if the resulting combination could not be validly published under Art. 32.1(c) or would be illegitimate under Art. 53, or (c) if Art. 11.7, 22.1, 26.1, or 59 rules that a different combination is to be used.
Ex. 6. Primula sect. Dionysiopsis Pax (in Jahresber. Schles. Ges. Vaterländ. Kultur 87: 20. 1909) when transferred to Dionysia Fenzl becomes D. sect. Dionysiopsis (Pax) Melch. (in Mitt. Thüring. Bot. Vereins 50: 164-168. 1943); the substitute name D. sect. Ariadna Wendelbo (in Bot. Not. 112: 496. 1959) is illegitimate under Art. 52.1.
Ex. 7. Antirrhinum spurium L. (1753) when transferred to Linaria Mill. is called L. spuria (L.) Mill. (1768).
Ex. 8. When transferring Serratula chamaepeuce L. (1753) to Ptilostemon Cass., Cassini illegitimately (Art. 52.1) named the species P. muticus Cass. (1826). In that genus, the correct name is P. chamaepeuce (L.) Less. (1832).
Ex. 9.The correct name for Rubus aculeatiflorus var. taitoensis (Hayata) T. S. Liu & T. Y. Yang (in Annual Taiwan Prov. Mus. 12: 12. 1969) is R. taitoensis Hayata var. taitoensis, because R. taitoensis Hayata (1911) has priority over R. aculeatiflorus Hayata (1915).
Ex. 10. When transferring Spartium biflorum Desf. (1798) to Cytisus Desf., Spach correctly proposed the substitute name C. fontanesii Spach (1849) because of the previously and validly published C. biflorus L'Hér. (1791); the combination C. biflorus based on S. biflorum would be illegitimate under Art. 53.1.
Ex. 11. Spergula stricta Sw. (1799) when transferred to Arenaria L. is called A. uliginosa Schleich. ex Schltdl. (1808) because of the existence of the name A. stricta Michx. (1803), based on a different type; but on further transfer to the genus Minuartia L. the epithet stricta is again available and the species is called M. stricta (Sw.) Hiern (1899).
Ex. 12. Arum dracunculus L. (1753) when transferred to Dracunculus Mill. is named D. vulgaris Schott (1832), as use of the Linnaean epithet would result in a tautonym (Art. 23.4).
Ex. 13. Cucubalus behen L. (1753) when transferred to Behen Moench was legitimately renamed B. vulgaris Moench (1794) to avoid the tautonym "B. behen". In Silene L., the epithet behen is unavailable because of the existence of S. behen L. (1753). Therefore, the substitute name S. cucubalus Wibel (1799) was proposed. This, however, is illegitimate (Art. 52.1) since the specific epithet vulgaris was available. In Silene, the correct name of the species is S. vulgaris (Moench) Garcke (1869).
Ex. 14. Helianthemum italicum var. micranthum Gren. & Godr. (Fl. France 1: 171. 1847) when transferred as a variety to H. penicillatum Thibaud ex Dunal retains its varietal epithet and is named H. penicillatum var. micranthum (Gren. & Godr.) Grosser (in Engler, Pflanzenr. 14: 115. 1903).
Ex. 15.The final epithet of the combination Thymus praecox subsp. arcticus (Durand) Jalas (in Veröff. Geobot. Inst. ETH Stiftung Rübel Zürich 43: 190. 1970), based on T. serpyllum var. arcticus Durand (Pl. Kaneanae Groenl. 196. 1856), was first used at the rank of subspecies in the combination T. serpyllum L. subsp. arcticus (Durand) Hyl. (in Uppsala Univ. Årsskr. 1945(7): 276. 1945). However, if T. britannicus Ronniger (1924) is included in this taxon, the correct name at subspecies rank is T. praecox subsp. britannicus (Ronniger) Holub (in Preslia 45: 359. 1973), for which the final epithet was first used at this rank in the combination T. serpyllum subsp. britannicus (Ronniger) P. Fourn. (Quatre Fl. France: 841. 1938, "S.-E. [Sous-Espèce] Th. Britannicus").
Note 1. The valid publication of a name at a rank lower than genus precludes any simultaneous homonymous combination (Art. 53), irrespective of the priority of other names with the same final epithet that may require transfer to the same genus or species.
Ex. 16. Tausch included two species in his new genus Alkanna: A. tinctoria Tausch (1824), a new species based on "Anchusa tinctoria" in the sense of Linnaeus (1762), and A. mathioli Tausch (1824), a nomen novum based on Lithospermum tinctorium L. (1753). Both names are legitimate and take priority from 1824.
Ex. 17. Raymond-Hamet transferred to the genus Sedum both Cotyledon sedoides DC. (1808) and Sempervivum sedoides Decne. (1844). He combined the epithet of the later name, Sempervivum sedoides, under Sedum as S. sedoides (Decne.) Raym.-Hamet (1929), and published a new name, S. candollei Raym.-Hamet (1929), for the earlier name. Both names are legitimate.
11.5. When, for any taxon of the rank of family or below, a choice is possible between legitimate names of equal priority in the corresponding rank, or between available final epithets of names of equal priority in the corresponding rank, the first such choice to be effectively published (Art. 29, 30, 31) establishes the priority of the chosen name, and of any legitimate combination with the same type and final epithet at that rank, over the other competing name(s) (but see Art. 11.6).
Note 2. A choice as provided for in Art. 11.5 is effected by adopting one of the competing names, or its final epithet in the required combination, and simultaneously rejecting or relegating to synonymy the other(s), or nomenclatural (homotypic) synonyms thereof.
Ex. 18. When Dentaria L. (1753) and Cardamine L. (1753) are united, the resulting genus is called Cardamine because that name was chosen by Crantz (Cl. Crucif. Emend.: 126. 1769), who first united them.
Ex. 19. When Claudopus Gillet (1876), Eccilia (Fr. : Fr.) P. Kumm. (1871), Entoloma (Fr. ex Rabenh.) P. Kumm. (1871), Leptonia (Fr. : Fr.) P. Kumm. (1871), and Nolanea (Fr. : Fr.) P. Kumm. (1871) are united, one of the generic names simultaneously published by Kummer must be used for the combined genus. Donk, who did so (in Bull. Jard. Bot. Buitenzorg, ser. 3, 18(1): 157. 1949), selected Entoloma, which is therefore treated as having priority over the other names.
Ex. 20. Brown (in Tuckey, Narr. Exped. Zaire: 484. 1818) was the first to unite Waltheria americana L. (1753) and W. indica L. (1753). He adopted the name W. indica for the combined species, and this name is accordingly treated as having priority over W. americana.
Ex. 21. Baillon (in Adansonia 3: 162. 1863), when uniting for the first time Sclerocroton integerrimus Hochst. (1845) and S. reticulatus Hochst. (1845), adopted the name Stillingia integerrima (Hochst.) Baill. for the combined taxon. Consequently Sclerocroton integerrimus is treated as having priority over S. reticulatus irrespective of the genus (Sclerocroton, Stillingia, Excoecaria or Sapium) to which the species is assigned.
Ex. 22. Linnaeus (1753) simultaneously published the names Verbesina alba and V. prostrata. Later (1771), he published Eclipta erecta, an illegitimate name because V. alba was cited in synonymy, and E. prostrata, based on V. prostrata. The first author to unite these taxa was Roxburgh (Fl. Ind., ed. 1832, 3: 438. 1832), who adopted the name E. prostrata (L.) L. Therefore V. prostrata is treated as having priority over V. alba.
Ex. 23. Donia speciosa and D. formosa, which were simultaneously published by Don (1832), were illegitimately renamed Clianthus oxleyi and C. dampieri, respectively, by Lindley (1835). Brown (in Sturt, Narr. Exped. C. Australia 2: 71. 1849) united both in a single species, adopting the illegitimate name C. dampieri and citing D. speciosa and C. oxleyi as synonyms; his choice is not of the kind provided for by Art. 11.5. Clianthus speciosus (G. Don) Asch. & Graebn. (1909), published with D. speciosa and C. dampieri listed as synonyms, is an illegitimate later homonym of C. speciosus (Endl.) Steud. (1840); again, conditions for a choice under Art. 11.5 were not satisfied. Ford & Vickery (1950) published the legitimate combination C. formosus (G. Don) Ford & Vickery and cited D. formosa and D. speciosa as synonyms, but since the epithet of the latter was unavailable in Clianthus a choice was not possible and again Art. 11.5 does not apply. Thompson (1990) was the first to effect an acceptable choice when publishing the combination Swainsona formosa (G. Don) Joy Thomps. and indicating that D. speciosa was a synonym of it.
11.6. An autonym is treated as having priority over the name or names of the same date and rank that established it.
Note 3. When the final epithet of an autonym is used in a new combination under the requirements of Art. 11.6, the basionym of that combination is the name from which the autonym is derived, or its basionym if it has one.
Ex. 24. By describing Synthyris subg. Plagiocarpus, Pennell (in Proc. Acad. Nat. Sci. Philadelphia 85: 86. 1933) established the name Synthyris Benth. subg. Synthyris (although using the designation "Eusynthyris"), and when this group is included in Veronica, V. subg. Synthyris (Benth.) M. M. Mart. Ort. & al. (in Taxon 53: 440. 2004) has precedence over a combination in Veronica based on S. subg. Plagiocarpus Pennell.
Ex. 25. Heracleum sibiricum L. (1753) includes H. sibiricum subsp. lecokii (Godr. & Gren.) Nyman (Consp. Fl. Eur.: 290. 1879) and H. sibiricum subsp. sibiricum automatically established at the same time. When H. sibiricum is included in H. sphondylium L. (1753) as a subspecies, the correct name for the taxon is H. sphondylium subsp. sibiricum (L.) Simonk. (Enum. Fl. Transsilv.: 266. 1887), not subsp. lecokii, whether or not subsp. lecokii is treated as distinct.
Ex. 26. The publication of Salix tristis var. microphylla Andersson (Salices Bor.-Amer.: 21. 1858) created the autonym S. tristis Aiton (1789) var. tristis, dating from 1858. If S. tristis, including var. microphylla, is recognized as a variety of S. humilis Marshall (1785), the correct name is S. humilis var. tristis (Aiton) Griggs (in Proc. Ohio Acad. Sci. 4: 301. 1905). However, if both varieties of S. tristis are recognized as varieties of S. humilis, then the names S. humilis var. tristis and S. humilis var. microphylla (Andersson) Fernald (in Rhodora 48: 46. 1946) are both used.
Ex. 27. In the classification adopted by Rollins and Shaw, Lesquerella lasiocarpa (Hook. ex A. Gray) S. Watson (1888) is composed of two subspecies, subsp. lasiocarpa (which includes the type of the name of the species and is cited without an author) and subsp. berlandieri (A. Gray) Rollins & E. A. Shaw. The latter subspecies is composed of two varieties. In that classification the correct name of the variety which includes the type of subsp. berlandieri is L. lasiocarpa var. berlandieri (A. Gray) Payson (1922), not L. lasiocarpa var. berlandieri (cited without an author) or L. lasiocarpa var. hispida (S. Watson) Rollins & E. A. Shaw (1972), based on Synthlipsis berlandieri var. hispida S. Watson (1882), since publication of the latter name established the autonym S. berlandieri A. Gray var. berlandieri which, at varietal rank, is treated as having priority over var. hispida.
11.7. For purposes of priority, names of fossil morphotaxa compete only with names based on a fossil type representing the same part, life-history stage, or preservational state (see Art. 1.2).
Ex. 28. The generic name Sigillaria Brongn. (1822), established for bark fragments, may in part represent the same biological taxon as the "cone-genus" Mazocarpon M. J. Benson (1918), which represents permineralizations, or Sigillariostrobus (Schimp.) Geinitz (1873), which represents compressions. Certain species of all three genera, Sigillaria, Mazocarpon, and Sigillariostrobus, have been assigned to the family Sigillariaceae. All these generic names can be used concurrently in spite of the fact that they may, at least in part, apply to the same organism.
Ex. 29. The morphogeneric name Tuberculodinium D. Wall (1967) may be retained for a genus of fossil cysts even though cysts of the same kind are known to be part of the life cycle of an extant genus that bears an earlier name, Pyrophacus F. Stein (1883).
Ex. 30. A common Jurassic leaf-compression fossil is referred to by different authors either as Ginkgo huttonii (Sternb.) Heer or Ginkgoites huttonii (Sternb.) M. Black. Both names are in accordance with the Code, and either name can correctly be used, depending on whether this Jurassic morphospecies is regarded as rightly assigned to the living (non-fossil) genus Ginkgo L. or whether it is more appropriate to assign it to the morphogenus Ginkgoites Seward (type, G. obovata (Nath.) Seward, a Triassic leaf compression).
11.8. Names of plants (diatoms excepted) based on a non-fossil type are treated as having priority over names of the same rank based on a fossil (or subfossil) type.
Ex. 31. If Platycarya Siebold & Zucc. (1843), a non-fossil genus, and Petrophiloides Bowerb. (1840), a fossil genus, are united, the name Platycarya is correct for the combined genus, although it is antedated by Petrophiloides.
Ex. 32. Boalch and Guy-Ohlson (in Taxon 41: 529-531. 1992) united the two prasinophyte genera Pachysphaera Ostenf. (1899) and Tasmanites E. J. Newton (1875). Pachysphaera is based on a non-fossil type and Tasmanites on a fossil type. Under the Code in effect in 1992, Tasmanites had priority and was therefore adopted. Under the current Code, in which the exemption in Art. 11.8 applies only to diatoms and not to algae in general, Pachysphaera is correct for the combined genus.
Ex. 33. The generic name Metasequoia Miki (1941) was based on the fossil type of M. disticha (Heer) Miki. After discovery of the non-fossil species M. glyptostroboides Hu & W. C. Cheng, conservation of Metasequoia Hu & W. C. Cheng (1948) as based on the non-fossil type was approved. Otherwise, any new generic name based on M. glyptostroboides would have had to be treated as having priority over Metasequoia Miki.
Note 4. The provisions of Art. 11 determine priority between different names applicable to the same taxon; they do not concern homonymy. In accordance with Art. 53, later homonyms are illegitimate whether the type is fossil or non-fossil.
Ex. 34. Endolepis Torr. (1861), based on a non-fossil type, is an illegitimate later homonym of, and does not have priority over, Endolepis Schleid. (1846), based on a fossil type.
Ex. 35. Cornus paucinervis Hance (1881), based on a non-fossil type, is an illegitimate later homonym and does not have priority over C. paucinervis Heer (Fl. Tert. Helv. 3: 289. 1859), based on a fossil type.
Ex. 36. Ficus crassipes F. M. Bailey (1889), F. tiliifolia Baker (1885), and F. tremula Warb. (1894), each based on a non-fossil type, were illegitimate later homonyms of, respectively, F. crassipes (Heer) Heer (1882), F. tiliifolia (A. Braun) Heer (1856), and F. tremula (Heer) Heer (1874), each based on a fossil type. The three names with non-fossil types have been conserved against their earlier homonyms in order to maintain their use.
11.9. For purposes of priority, names in Latin form given to hybrids are subject to the same rules as are those of non-hybrid taxa of equivalent rank.
Ex. 37. The name ×Solidaster H. R. Wehrh. (1932) antedates ×Asterago Everett (1937) for the hybrids between Aster L. and Solidago L.
Ex. 38. Anemone ×hybrida Paxton (1848) antedates A. ×elegans Decne. (1852), pro sp., as the binomial for the hybrids derived from A. hupehensis (Lemoine & É. Lemoine) Lemoine & É. Lemoine × A. vitifolia Buch.-Ham. ex DC.
Ex. 39. Camus (in Bull. Mus. Natl. Hist. Nat. 33: 538. 1927) published the name ×Agroelymus A. Camus for a nothogenus, without a Latin description or diagnosis, mentioning only the names of the parents involved (Agropyron Gaertn. and Elymus L.). Since this name was not validly published under the Code then in force, Rousseau (in Mém. Jard. Bot. Montréal 29: 10-11. 1952) published a Latin diagnosis. However, the date of valid publication of ×Agroelymus under this Code (Art. H.9) is 1927, not 1952, so it antedates the name ×Elymopyrum Cugnac (in Bull. Soc. Hist. Nat. Ardennes 33: 14. 1938).
11.10. The principle of priority does not apply above the rank of family (but see Rec. 16B).

References: Art. 18
 Art. 1
 Art. 53
 Art. 14
 Art. 11
 Art. 14
 Art. 32
 Art. 53
 Art. 11
 Art. 52
 Art. 53
 Art. 11
 Art. 11
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 V. 
 V. 
 V. 
 V. 
 Art. 11
 Art. 11
 Art. 11
 Art. 11
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 Art. 1
 Art. 11
 Art. 11
 Art. 53