The Sindarin word for a “twin”, with a plural form gwenyn using the ordinary pluralization rules of Sindarin (PE23/140; PM/365). It is likely a combination of gwa- “together” and primitive *nonā “born” based on the root √NŌ; compare Q. onóna “twin-born” of similar derivation (WJ/367). It also had a specialized dual form gwanûr for a pair of twins (LotR/1054), derived instead from ✶wo-nōrē “together-birth”, functionally a singular noun in archaic speech but in modern speech treated as a variant plural applicable only to a pair of twins (PE23/140). Gwanûr is thus probably used more often than gwenyn.

Conceptual Development: The Gnomish Lexicon of the 1910s had G. gadwi “twins”, a dual form of G. gad(a) “link” (GL/37), along with a longer variant G. hethgadwi or heth-gedwi that had heth “sibling” as a prefix (GL/49). The Quendi and Eldar (Q&E) essay from 1959-60 had another dual form gwanūn, probably based on *wo-nōnā, from which a singular gwanunig was derived.

Neo-Sindarin: For purposes of Neo-Sindarin, I would use gwanon as the singular, gwanûr as the dual, and gwenyn as the ordinary plural, since these forms appear in either The Lord of the Rings (LotR/1054) or in notes from 1969 (PE23/140; PM/365).