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Trinomial nomenclature
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Trinomial nomenclature

Trinomial nomenclature is a taxonomic naming system that extends the standard system of binomial nomenclature by adding a third epithet. It is used in biology when the organisms within a species fall into separate groupings that need to be distinguished.

The hierarchy of taxa below the species level is:

Not all are necessarily used for any particular species. In many cases the sub specific groupings are regional. Because of geographical isolation and adaptation to the different circumstances the regional population evolves and over time may become a separate species.

The simplest form of trinomial nomenclature occurs when only a subspecies is being specified. In this case, the subspecies name is added after the species name, without capitalisation but normally typeset in italics. If the genus and species name have already been mentioned in the same paragraph, they are often abbreviated to initial letters: for example one might write, "The Great Cormorant Phalacrocorax carbo has a distinct subspecies in Australasia, the Black Shag P. c. novaehollandiae".

If taxa below the subspecies level are used (which most often arises with plants), an identifying abbreviation is always added. In groups where such lower level divisions are common, it is wiser to indicate when a subspecies name is used, also.

From a scientific point of view, a name is incomplete without an author label and publication details. This specifies who published the name and in what publication and gives the date of the publication in which the name was published.