In biology, trinomial nomenclature refers to the use of three-term scientific names assigned to taxa ranked below the species level. The phrase literally means "three-part naming". It is most commonly applied to subspecies, varieties, forms, or other infraspecific categories depending on the group of organisms and the code of nomenclature that governs them.
Structure and components
A trinomial name consists of three successive parts. The first part is the genus name, which is capitalized; the second is the species epithet, which is not capitalized; the third is the infraspecific or subspecific epithet, also not capitalized. In printed works these elements are typically italicized to indicate their Latinized scientific status. Example general format: Genus species subspecies.
Rules and variation by discipline
Different codes of nomenclature set the formal rules for forming and using trinomials. For animals the International Code of Zoological Nomenclature (ICZN) recognizes subspecies and trinomials without an intervening rank indicator, while botanical and mycological practice governed by the International Code of Nomenclature for algae, fungi and plants (ICN) often inserts abbreviations such as "subsp." or "var." to show the rank explicitly. Typical conventions include:
- Capitalize only the genus name; keep the other epithets lowercase.
- Italicize the whole name in typeset text (or underline if handwriting).
- Include an author citation after the name when precision in nomenclature is required.
Examples and common uses
Trinomials are used when populations within a species are distinct enough to be recognized formally but not separate species. Well-known examples include Gorilla gorilla gorilla (western lowland gorilla) and Panthera tigris altaica (the Siberian tiger subspecies). Domestic animals and widely studied wild taxa often have named subspecies or varieties that are written as trinomials.
Importance and notable points
Using trinomials helps biologists document geographic variation, conservation units, and evolutionary relationships within species. However, not all fields use trinomials equally: microorganisms, plants, animals and viruses follow different conventions and some taxonomists prefer to recognize distinct population units as separate species rather than subspecies. The practice thus reflects both biological reality and the taxonomic judgment applied by specialists.