The term "xxi," as the Roman numeral for twenty-one, primarily functions as an adjective. Specifically, it is an ordinal adjective used to denote the twenty-first position in a sequence. Its function is to modify a noun, providing information about its specific order or identity within a series, such as a century, a monarchical line, or an annual event.
In grammatical application, "xxi" is most often a postpositive adjective, meaning it is placed directly after the noun it modifies. Common examples include "the XXI century," "Pope John XXI," or "Super Bowl XXI." In each case, "xxi" specifies which century, pope, or event is being referenced. While it can also function as a noun when referring to the number or symbol itself (e.g., "The Roman numeral for twenty-one is XXI"), its most prevalent and contextually significant role in formal writing and titles is adjectival.
For the purpose of an article, establishing "xxi" as an adjective is crucial for ensuring syntactic correctness. This determination dictates that the term must be associated with a noun to form a coherent phrase. The main point of the article will therefore not be about the numeral in isolation, but rather about the specific entity it qualifies, such as the art, politics, or technology of the 21st (XXI) century. Treating it as an adjective frames the subject matter correctly from the outset.
