Why does a subspecies name contain three parts instead of two?

In biological classification, the standard system for naming species is binomial nomenclature, which uses two parts: the genus and the species. A subspecies name contains three parts—a system known as trinomialism—to provide more specific taxonomic detail.1234

The third part, or subspecies epithet, is added to indicate that an organism is a distinct population within a more widely distributed species. This is often used when a species is polytypic, meaning it consists of multiple geographically or genetically isolated groups that are still considered part of the same species. For example, the domestic dog is classified as Canis lupus familiaris; the third term, familiaris, identifies it as a specific descendant of the gray wolf (Canis lupus).42

By using a trinomial designation, scientists can clearly communicate that an organism is a local representative or a specific variant of a broader, established species.4