Rules for assigning scientific names have become well codified in order to keep the names internationally unambiguous and understandable. The full set of rules is rather involved, but the most important parts are fairly simple:

Rules for Zoology

• Binomens - A genus name is one word. A species name is binomial -- the genus plus a second word. Subspecies have a trinomial name (a "trinomen"). A subgenus is occasionally given in parentheses after the genus, thus:

• Authorship - The author's name and date of publication are typically given after the scientific name. If a name is later changed (e.g., moved to a new genus), The original author is given in parentheses. The names are often abbreviated; in particular, "L." is Linnaeus. Anonymous publication is invalid as of 1950, but was accepted before then.

• Description - After 1930, new names must come with a description (or reference to one) telling what the name means.

• Type specimen - Descriptions should refer to an actual specimen, available for examination in a museum or other collection. There are complicated rules for determining the type if the original is lost or if there was no type specimen with the original description. The taxonomy of types is rather complicated in itself; see A Compendium of Zoological Type Nomenclature: a Reference Source by Neal L. Evenhuis, Bishop Museum Technical Report 41 (2008).
  
  In a 1959 article on Linnaeus, William Stearn wrote, "Linnaeus himself, must stand as the type of his Homo sapiens." Though it was an off-hand comment, it suffices as nomenclatural act, and so Linnaeus, buried beneath Uppsala Cathedral, is the lectotype (a later-selected type specimen) for Homo sapiens. A 1994 publication reported that Robert Bakker proposed to designate Edward Cope as the type, but Bakker never actually published this. Besides, Stearn's designation has priority.

• Priority - The oldest valid published name is the one that gets used. Today one need not look back further than the works of Linnaeus, but Linnaeus and his students searched older works for the oldest published names, going back even to Thucydides and Pliny the Elder. Early publication is not invalidated even if there is some error in the original name:

There are means of overruling priority, if the newer name has come into common use before the priority of the original name is recognized.

Sometimes the same name gets reused by people not aware of the original use:

However, the same name can be used for a plant and an animal. There are hundreds of instances. See hemihomonyms for the long list (though it includes invalid names). The samples given below are those I found before discovering that website.

There are even a few cases of duplicated binomials, where both genus and species get reused in different kingdoms:

• Words and Letters - The names must be pronouncible words (preferably Latinized), using Latin letters, with no diacritics or punctuation (except hyphens can be used in some circumstances).
  
  Some names stretch the pronouncibility.

There are no official rules about how names should be pronounced. Still, many names have right and wrong pronunciations according to conventional usage.

• Exceptions - When an otherwise valid name would "disturb stability or universality or cause confusion," the International Commission on Zoological Nomenclature may choose another name instead.

Rules for Botany

The rules above are for zoological nomenclature. The rules for botanical nomenclature are similar. Some exceptions are:

• For valid publication, after Jan. 1, 1935 (1958 for algae, 1996 for fossils), the plant name must be accompanied by a Latin description or diagnosis, or by a reference to one.
• Publication of species or lower ranks must, after 1912 (1958 for algae), be accompanied by an illustration showing essential characters, or by a reference to one. After 2001, one such illustration must represent the type specimen.
• The authors are given in taxonomic monographs, and if a name is changed, both the original author (in parentheses) and the revising author are named. For example:

• Zoology allows tautonyms (genus and species repeating the same word, e.g. Bufo bufo), and botany does not. Some names come close, though.

• Zoologists do not like hyphens in names, although a very few have snuck in:
  Zygiella x-notata (Clerck 1757) (silver-sided sector spider)
  Polygonia c-album (Linnaeus 1758) (comma butterfly)
  Xestia c-nigrum (Linnaeus 1758) (the Setaceous Hebrew character (a moth))
  Astroscopus y-graecum (Cuvier 1829) (southern stargazer fish)
  Somateria mollissima v-nigrum Bonaparte, 1855 (Pacific eider)
  Phelsuma v-nigra (Boettger 1913) (gecko)
  Botany does not encourage hyphens, but allows them more freely:

• Rules for botany (at least since 1912) forbid generic names which refer to morphological characters.
• The International Code of Nomenclature for algae, fungi, and plants goes into great length concerning the messy subject of hybrids. Orchids present enough additional difficulties in this area that they have their own Handbook on Orchid Nomenclature and Registration.
• Botany allows names for ranks below subspecies: variety (var.), subvariety (subvar.), form (f.), and subform (subf.). These need not be nested within a subspecies or each other.

Rules for Fungi

Traditionally, fungi have been grouped with plants (even though, as later discovered, they are more closely related to animals). In 2011, this grouping was formalized, with the botanical code (ICBN) renamed the International Code of Nomenclature for algae, fungi, and plants (ICN).

Because fungi often have very different sexual and asexual forms, the same species of fungus could have two different scientific names, one for the sexual form and one for the asexual. This rule, however, has changed (with much gnashing of teeth) as of 2011, because DNA analysis makes it easier to recognize two forms as the same species.

Rules for Bacteria

There is also a separate International Code of Nomenclature of Prokaryotes governed by the International Committee on Systematics of Prokaryotes (ICSP). It has published the Approved Lists of Bacterial Names, listing all names valid as of 1 January 1980; it serves as a starting point for adding new names. "(Approved Lists 1980)" may be used in place of an author citation for names on the lists. Other rules particular to prokaryotes are:

• Names must be published in International Journal of Systematic Bacteriology or International Journal of Systematic and Evolutionary Microbiology to be valid.
• Subgenus and subspecies names, when present, are labelled "subgen." and "subsp."
• Prokaryote names must avoid existing names of plants and animals. However, there are several exceptions where a bacterial genus shares it name with a plant (or fungus), animal, or both. Here are cases where the name covers three phyla (listing bacterium, animal, plant/fungus in that order).

• The Code recommends that if a species is named after a person, the person should in some way be connected with it.
• A name should be rejected "whose application is likely to lead to accidents endangering health or life or both or of serious economic consequences."

See also Trüper 1999 in the references.

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