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Unambiguous names for organisms are essential for effective scientific communication; names can only be unambiguous if there are internationally accepted rules governing their formation and use. The rules that govern scientific naming in botany (including phycology and mycology) are revised at Nomenclature Section meetings at successive International Botanical Congresses. The present edition of the International code of botanical nomenclature embodies the decisions of the XVII International Botanical Congress held in Vienna in 2005 and supersedes the Saint Louis Code, published six years ago subsequent to the XVI International Botanical Congress in St Louis, Missouri, U.S.A. It is written entirely in (British) English. The St Louis Code was translated into Chinese, French, Japanese, Portuguese, Russian, Slovak, and Spanish; it is therefore anticipated that the Vienna Code, too, will become available in several languages in due course.

One of the reasons invoked for the choice of Vienna as the site of the seventeenth Congress, was that the second International Botanical Congress had been held there exactly 100 years earlier. It was that Congress that accepted the first internationally developed rules governing the naming of plants, Règles internationales de la Nomenclature botanique adoptées par le Congrès International de Botanique de Vienne 1905 / International rules of Botanical Nomenclature ... / Internationale Regeln der Botanischen Nomenclatur ... – or simply the Vienna Rules, thus obviating confusion with this Vienna Code, and not requiring this Code to bear any qualifying numeral.

The Vienna Code does not differ substantially in overall presentation and arrangement from the St Louis Code, and the numbering of Articles remains the same, although there have been a few additions to, and modifications of, paragraphs, Recommendations, and Examples, often involving changes in their numbering. One small change has also been made in the numbering of the Appendices to make this more logical: the former App. IIIA, dealing with conserved names of genera is now simply App. III, and the former App. IIIB, with names of species, becomes App. IV. With App. IIA & IIB continuing to contain the two sorts of conserved family names, there is now a logical sequence for the lists of conserved names: II for families, III for genera, and IV for species. The subsequent Appendices increase in number accordingly, so that names rejected “utique” under Art. 56 form App. V, and suppressed works, App. VI. The St Louis Code omitted the “Important Dates in the Code” that had appeared in the Berlin & Tokyo Codes; this has been restored in the Vienna Code, from a draft by D. L. Hawksworth.

In overall presentation the most notable feature, however, is the inclusion for the first time of a Glossary, which appears as Appendix VII. This was requested by the Vienna Congress and it was made clear that it should be an integral part of the Code with all the authority thereof. This has meant that the Glossary is very tightly linked to the wording of the Code, and only nomenclatural terms defined in the Code can be included. A few other terms in more general use and not defined in the Code (e.g. description, position, rank) but with distinctive application in the Code have, however, also been included; they are distinguished by the statement “not defined” followed by an explanation of the way in which, in the opinion of the Editorial Committee, they are applied in the Code. For the preparation of the Glossary, the Committee is particularly grateful to P. C. Silva, who initiated the project and who prepared the first draft for consideration by the Editorial Committee and who has worked over several subsequent ones, ensuring precision and consistency.

The text of the Code uses three different sizes of print, the Recommendations and Notes being set in smaller type than the Articles, and the Examples and footnotes in smaller type than the Recommendations and Notes. The type sizes reflect the distinction between the rules which are mandatory (Articles), complementary information or advice (Notes and Recommendations), and explanatory material (Examples and footnotes). A Note has binding effect but does not introduce any new provision or concept; rather, it explains something that may not at first be readily apparent but is covered explicitly or implicitly elsewhere in the Code. Some Examples, which were deliberately agreed by a Nomenclature Section, contain material which is not fully, or not explicitly, covered in the rules. Such “voted examples” are prefixed by an asterisk (*). If, by a change of the corresponding provision in a subsequent edition of the Code, a “voted example” becomes fully covered, the asterisk is removed.

As in the previous edition, scientific names under the jurisdiction of the Code, irrespective of rank, are consistently printed in italic type. The Code sets no binding standard in this respect, as typography is a matter of editorial style and tradition not of nomenclature. Nevertheless, editors and authors, in the interest of international uniformity, may wish to consider adhering to the practice exemplified by the Code, which has been well received in general and is followed in a number of botanical and mycological journals. To set off scientific plant names even better, the abandonment in the Code of italics for technical terms and other words in Latin, traditional but inconsistent in early editions, has been maintained.

Like its forerunners, the Editorial Committee has tried hard to achieve uniformity in bibliographic style and formal presentation – a sound educational exercise for its members, and a worthwhile goal because the Code is considered a model to follow by many of its users. The titles of books in bibliographic citations are abbreviated in conformity with Taxonomic literature, ed. 2, by Stafleu & Cowan (1976-1988; with Supplements 1-6 by Stafleu & Mennega, 1992-2000), or by analogy, but with capital initial letters. For journal titles, the abbreviations follow the Botanico-periodicum-huntianum, ed. 2 (2004).

Author citations of scientific names appearing in the Code are standardized in conformity with Authors of plant names, by Brummitt & Powell (1992), as mentioned in Rec. 46A Note 1; these are also adopted and updated by the International Plant Names Index, and may be accessed at One may note that the Code has no tradition of recording the ascription of names to pre-1753 authors by the validating author, although such “pre-ex” author citations are permitted (see Art. 46 Ex. 33).

Like its immediate predecessor, the Vienna Congress was conservative in nomenclatural matters in comparison with some earlier Congresses. Relatively few changes were accepted, but a small number of significant ones and many useful clarifications and improvements of the Code, both in wording and substance, were adopted. Here we only draw attention to changes of some note. A full report on the Section’s decisions has been published elsewhere (McNeill & al. in Taxon 54: 1057-1064. 2006).

Perhaps the most important single decision incorporated into the Vienna Code was to deal with what many have recognized as a bomb waiting to explode, the publication status of theses submitted for a higher degree. In most, but certainly not all, countries, such theses have not traditionally been considered media for effective publication under the Code, and degree candidates have normally gone on to publish in journals or monograph series the taxonomic novelties and nomenclatural actions contained in their theses. However, as soon as theses ceased to be typewritten with carbon copies, or as soon as they were made available commercially by photo-reproduction, no provision existed in the Code to treat them as other than effectively published. Because of the fact that in some other countries, notably the Netherlands and some Scandinavian countries, theses, to be accepted, must be produced in substantial numbers and are intended as effectively published media, it has not hitherto been possible to resolve the issue. Nevertheless, despite the lack of any justification in the Code for treating most theses produced over the past 40 years as other than effectively published, the practice not to do so has persisted outside of a few countries. In consequence, the Section took the unusual step of accepting a retroactive change in the Code by deciding that no independent non-serial publication stated to be a thesis submitted for a higher degree on or after 1 January 1953 would be considered an effectively published work without a statement to that effect or other internal evidence. The Editorial Committee was instructed to provide examples of internal evidence that would best reflect current practice. The new Art. 30 Note 2 refers to the presence of an International Standard Book Number (ISBN) or a statement of the name of the printer, publisher, or distributor in the original printed version as such internal evidence.

Several proposals on criteria for valid publication of names were considered in Vienna. It was made explicit that names be composed only of letters of the Latin alphabet except as otherwise provided in the Code, and some clarification was accepted on what constitutes a description or diagnosis: statements on usage of plants, on cultural and cultivation features, and on geographical origin or geological age are not acceptable, nor is the mere mention of features but not their expression. Conceptually more significant, however, was the decision to make provision for binding decisions on whether or not a descriptive statement meets the requirement of Art. 32.1(d) for a “description or diagnosis” – the so-called “nomina subnuda” situation. This introduces into the Code an entirely new concept in botanical nomenclature, although one that is well-established in zoological nomenclature, namely rulings on interpretation of the Code itself. Since the Sydney Congress of 1981, there has been provision for rulings on whether or not two names or epithets are likely to be confused, and, of course, in the conservation and rejection of names, judgement must be made as to whether or not there will be “disadvantageous nomenclatural change”, but these do not involve interpretation of the Code itself. The procedure established is the same as that for judgement on whether names or epithets are sufficiently alike to be confused (Art. 53.5) and the General Committee will probably need to establish mechanisms to ensure that proposed rulings coming from the different Permanent Committees are reasonably consistent in their interpretation of Art. 32.1(d).

Article 33, dealing with new combinations, although improved significantly at the previous Congress, was again the subject of clarification, principally in making a separation in the paragraphs of the Article between the situation before 1 January 1953 and the more precise requirements from that date onward. In addition it was made clearer that, prior to 1 January 1953, when the epithet of a previously and validly published name that applies to the same taxon is adopted, the “presumed new combination” is validly published if there is any indication at all of a basionym, however indirect, but if there is no such indication, the new combination is only validly published if it would otherwise be a validly published name. By contrast, it was accepted in Vienna that on or after 1 January 1953 a claimed new combination or avowed substitute, that lacks the full information required regarding the basionym or replaced synonym is not validly published even though the name would otherwise be validly published as the name of a new taxon. Although involving the somewhat cumbersome expression “a generic name with a basionym” it has been made explicit that most of the rules on combinations apply also to such generic names.

Three important sets of changes were accepted in Vienna applying to names in particular groups of organisms, fossil plants, pleomorphic fungi, and fungi that had previously been named under the ICZN, respectively.

That for fossil plants was a reversal of one component of the rules on morphotaxa introduced in the St Louis Code. At the St Louis Congress it was argued (and accepted) that all fossil taxa should be treated as morphotaxa. This has not, however, been considered appropriate by the majority of palaeobotanists and a distinction between a morphotaxon and a regular fossil taxon is now established. Whereas a morphotaxon comprises only the one part, life-history stage, or preservational state represented by the type of its name, any new fossil taxon that is described as including more than one part, life-history stage, or preservational state is not a morphotaxon. A corollary of this change is that Art. 11.7 of the Tokyo Code has had to be reinstated (as Art. 11.8 of the Vienna Code) because priority of a name of a taxon based on a non-fossil type competing with one for the same taxon based on a fossil type is no longer implicit. Opportunity has also been taken to make clear that later homonyms are illegitimate whether the type is fossil or non-fossil.

The Code has long provided for a dual nomenclature for fungi with a pleomorphic life history. Proposals to amend the article involved (Art. 59) in order to facilitate a single name for a fungal taxon for which the anamorph-teleomorph relationship is known were extensively debated amongst the mycologists present in Vienna who came to a consensus on one very significant change in Art. 59, through which, by using the epitype concept, a name, currently only applicable to an anamorph, may be applied in the future to the whole organism (the holomorph) – cf. Art. 59.7.

A very important change in the Code, as it affects certain groups of organisms now recognized as fungi, is the extension to fungi of the provision of the second sentence of Art. 45.4, previously applicable only to algae. This deals with the names of taxa originally assigned to a group not covered by the ICBN, but which are now considered to be either algae – or now also fungi. To be accepted as validly published under the ICBN, such names need only meet the requirements of the pertinent non-botanical Code. The particular situation that triggered the proposal was that of the Microsporidia, long considered protozoa and now recognized as fungi. In addition, species names in the genus Pneumocystis (Archiascomycetes), containing important human and other mammalian pathogens, none of which were validly published under the St Louis Code (usually because of the lack of a Latin diagnosis or description), are now also to be treated as validly published. The change may have negative effects on a few names in groups longer established as fungi such as slime moulds, labyrinthulids, and trichomycetes, at least on authorship, but the numbers and importance are considered small compared with the benefits for the microsporidians and the species of Pneumocystis.

In the St Louis Code, the previously rather ambiguous restrictions on illustrations as types of names published after the type method entered the Code were clarified by establishing that illustrations were permitted as types of names published before 1 January 1958, but were prohibited thereafter unless it were “impossible to preserve a specimen”, a condition that many felt hard to define. Many at the Vienna Congress also felt that this “clarification” had had the effect of retroactively devalidating names published after 1957 with an illustration as type. The Congress agreed to move the date and decided that for names of microscopic algae and microfungi for which preservation of a type was technically difficult, the type might be an illustration, but that for all other organisms, names published on or after 1 January 2007 would require a specimen as type.

Stemming from the Report of the Special Committee on Suprageneric Names set up at the St Louis Congress, it was agreed that the starting date for valid publication of suprageneric names of spermatophytes, pteridophytes, and bryophytes (excluding those mosses already with a 1801 starting date) be 4 August 1789, the date of publication of Jussieu’s Genera plantarum. This restores the original basis of spermatophyte family names in App. IIB, dating to the Montreal Congress of 1959, which had never been included in any article of the Code, and which had had to be changed in the St Louis Code as a result of the Tokyo Congress failing to support a proposal similar to this one and the St Louis Congress deleting a protecting footnote. The Section also established that parenthetic author citation is not permitted at suprageneric ranks.

Full details of unavoidable changes made to Appendix IIB since the St Louis Code were published in the Second Report of the Special Committee on Suprageneric Names (Turland & Watson in Taxon 54: 491-499. 2005). The amendment to Art. 18.2, new Note 1 and voted Ex. 4, accepted at the Vienna Congress, have necessitated some additional changes since that Committee’s report and it is appropriate to detail these here. When, in a work, taxa ranked as orders are subdivided into families, the names of those taxa must be treated at the stated ranks and the orders cannot be treated as having been published as families under Art. 18.2. The orders and families in Berchtold & Presl’s O přirozenosti rostlin (1820) were already treated at the stated ranks, although Ambrosiaceae and Asteraceae, previously listed from Martinov’s Tekhno-botanicheskii Slovar (1820) have been updated because Berchtold & Presl published their book earlier in 1820 (Jan-Apr) than Martinov (3 Aug) (A. Doweld, pers. comm.). In Vines’s A student’s text-book of botany (1895) one order is subdivided into families, two of which, Cymodoceaceae and Posidoniaceae, have been updated. Six names in Link’s Handbuch, vols. 1 and 2 (1829) have been updated because, in vol. 3, Link published two family names under the order Fungi, which means that the names ranked as orders throughout the work (Art. 35.5) must be treated as the names of orders, not as families as has traditionally been done. The affected names are Dodonaeaceae, Melianthaceae, Moraceae, Neuradaceae, Tetragoniaceae and Theophrastaceae. In addition, Cordiaceae, which was updated to Link in the Special Committee’s report, remains as listed in App. IIB in the St Louis Code. Moreover, four family names previously overlooked in Berchtold & Presl’s rare, later, multi-volume work of the same name (1823-1825) have been updated: Aquifoliaceae, Cornaceae, Potamogetonaceae and Punicaceae.

The rules determining when a rank is denoted by a misplaced term (and hence not validly published) were clarified and made more practical. This introduced the concepts of “minimum invalidity” (Art. 33.10), by which only those names with rank-denoting terms that must be removed to provide the correct sequence would be considered not validly published, and of “informal usage” (Art. 33.11), for situations in which the same term was used for several different non-sequential ranks; such names are to be treated as validly published but unranked. It was established that having the ranks of both order and family in a work precluded application of Art. 18.2 (and similarly Art. 19.2 in the cases of suborder and subfamily), and that sequential use of the same rank did not preclude valid publication (Art. 33 Note 3).

One further date limit first appears in the Vienna Code. From 1 January 2007 a new combination, a new generic name with a basionym, or an avowed substitute is not validly published unless its basionym or replaced synonym is cited. Currently, although a full and direct reference to the place of publication must be given, the basionym or replaced synonym need only be indicated.

One portion of the Code that remains virtually unchanged after Vienna is that for which by far the largest number of amendment proposals (147) was submitted, namely orthography Of the 147 proposals, only five were accepted but 107 were referred to the Editorial Committee. After review of all these proposals by a subcommittee of the Editorial Committee (F. R. Barrie, D. H. Nicolson, and N. J. Turland, who gratefully acknowledge advice from R. Gereau, Missouri Botanical Garden), the changes incorporated into the Vienna Code are very few and none imposes a significant change in practice. The most notable is a clarification of the respective application of Rec. 60C.1 and 60C.2.

The Code establishes (Art. 12.1) that only if validly published does a name have any status; indeed, unless otherwise indicated, the word “name” in the Code means a name that has been validly published (Art. 6.3). For this reason recent editions of the Code have replaced “name” by “designation” when the requirements for valid publication have not been met, and the Vienna Code has taken this further by avoiding such contradictory expressions as a name being validated, or being invalid. Given the very different meaning of “valid” and “invalid” applied to names in zoological nomenclature (equivalent to the botanical “correct” and “incorrect”), it is convenient that neither “valid name” nor “invalid name” need be used in botanical nomenclature: either a name is validly published or else it is not a validly published name, i.e. not a name under the Code.

The Vienna Code was prepared according to the procedures outlined in Div. III, which have been operating with hardly any change since the Paris Congress of 1954. A total of 312 individual numbered amendment proposals were published in Taxon between February 2002 and November 2004. Their synopsis, with comments by the Rapporteurs, appeared in Taxon (54: 215-250) in February 2005 and served as the basis for the preliminary, non-binding mail vote by the members of the International Association for Plant Taxonomy (and some other persons), as specified in Division III of the Code. Tabulation of the mail vote was taken care of by the Nomenclature Section’s Recorder, T. F. Stuessy and his assistants in Vienna. The results were made available to the members of the Nomenclature Section at the beginning of its meetings; they were also tabulated in the November 2005 issue of Taxon (54: 1057-1064), along with the action taken by Congress.

The Nomenclature Section met at the Uni-Campus, University of Vienna, Spitalgasse 2, Vienna, on 12-16 July 2005. With 198 registered members carrying 402 institutional votes in addition to their personal votes, the Vienna Section had a large attendance compared with many previous Congresses but was substantially smaller than that at St Louis (with 297 members carrying 494 institutional votes). The Section Officers, previously appointed in conformity with Division III of the Code, were D. H. Nicolson (President), T. F. Stuessy (Recorder), J. McNeill (Rapporteur-général), and N. J. Turland (Vice-Rapporteur). Each Nomenclature Section is entitled to define its own procedural rules within the limits set by the Code, but tradition is held sacred. As on previous occasions, at least a 60% assenting majority was required for any proposed change to the Code to be adopted. Proposals that received 75% or more “no” votes in the mail ballot were ruled as rejected unless raised anew from the floor. The proceedings of the nomenclature sessions are presently being edited, based on a tape transcript. They will be published later this year or early in 2007 in the serial Englera.

The Nomenclature Section also appointed the Editorial Committee for the Vienna Code. As is traditional, only persons present at the Section meetings were invited to serve on that Committee, which as the Code requires is chaired by the Rapporteur-général and as is logical includes the Vice-Rapporteur as its secretary. The Editorial Committee sadly lost one of its members, when Guanghua Zhu died on 2 November 2005; the other 12 members of the committee convened on 6 January 2006 at the Missouri Botanical Garden, St Louis, U.S.A., for a full week’s hard work. The Committee worked on the basis of a draft of the text of the main body of the Code, prepared by the Chairman to incorporate the changes decided by the Section, which was distributed by electronic mail in December 2005; and of a preliminary version of the proceedings of the Section meetings, as transcribed from tape and revised portion-wise by F. R. Barrie, D. L. Hawksworth, J. McNeill, D. H Nicolson, and N. J. Turland.

Each Editorial Committee has the task of addressing matters specifically referred to it, incorporating changes agreed by the Section, clarifying any ambiguous wording, ensuring consistency, and providing additional examples for inclusion. The terms of the Committee’s mandate, as defined by the Section in Vienna at its constituent meeting, included the usual empowerment to alter the wording, the examples, or the location of Articles and Recommendations, in so far as the meaning was not affected; while retaining the present numbering in so far as possible.

The full Editorial Committee concentrated on the main body of the Code, including Appendix I (hybrids). A new electronic draft of these portions was completed prior to the end of its meeting, and provided to all Committee members for checking and for any further necessary clarification; as a result a revised draft was prepared and circulated in mid-May to all members for final proofreading. The contents of Appendices II-VI were revised and updated in a bilateral process involving the Chairman and a specialist for each of the groups concerned, normally a Committee member (V. Demoulin for the fungi, D. H. Nicolson for genera and species of vascular plants, P. C. Silva for the algae, J. E. Skog for fossil plants, N. J. Turland for family names of vascular plants), except for the bryophytes (G. Zijlstra, Utrecht, Secretary, Committee for Bryophyta, with assistance from P. Isoviita, Helsinki). The Secretaries of the other Permanent Committees for particular groups provided useful assistance to the responsible Editorial Committee member. The Subject index and the Index to scientific names were revised by J. Prado; the Index to the Appendices was updated by J. McNeill, who, with N. J. Turland, also cared for the final copy-editing; the time-consuming task of final formatting and production of camera-ready copy was carried out by N. J. Turland.

This is the proper place for us to thank all those who have contributed to the publication of the new Code: our fellow members of the Editorial Committee for their forbearance, helpfulness, and congeniality; all the persons, just named, who contributed in a special way and much beyond their normal commitment to particular editorial tasks; the botanists at large who volunteered advice and suggestions, including relevant new examples; the International Association for Plant Taxonomy and its Secretary, Tod Stuessy, for maintaining IAPT’s traditional commitment to plant nomenclature by funding travel and some ancillary costs for the Editorial Committee meeting in St Louis; the Missouri Botanical Garden and its Director, Peter Raven, for providing accommodation free of charge and hospitality for that meeting; and the publisher, Sven Koeltz, for his helpfulness and the speed with which he once again guided the Code through the printing process.

In addition to those who have helped to make possible this new edition of the Code, botanical nomenclature depends on the scores of botanists who serve on the Permanent Nomenclature Committees that work continuously between Congresses, dealing principally with proposals for conservation or rejection of names, and also those who are members of Special Committees set up by the Nomenclature Section of the Congress to review and seek solutions to particular nomenclatural problems. Botanical nomenclature is remarkable for the large number of taxonomists who voluntarily work so effectively and so extensively to the immeasurable benefit of all those who use plant names. On their behalf we express our sincere thanks to all who participate in this work.

The International Code of Botanical Nomenclature is published under the ultimate authority of the International Botanical Congresses. Provisions for the modification of the Code are detailed in Division III (p. 117). The next International Botanical Congress will be held in Melbourne, Australia from 23-30 July 2011, with a Nomenclature Section meeting likely in the preceding week. Invitation for proposals to amend this Code and instructions on procedure and format will be published in Taxon during 2007.

Like other international codes of nomenclature the ICBN has no legal status and is dependent on the voluntary acceptance of its rules by authors, editors, and other users of plant names. We trust that this Vienna Code will make their work just that little easier.

Edinburgh & St Louis, 24 July 2006

John McNeill

Nicholas J. Turland

(c) 2006, by International Association for Plant Taxonomy. This page last updated 26.02.2007 .