Like Quenya, Sindarin once had a dual inflection, used for pairs of objects, but in Sindarin this dual has fallen out of use:
The S. duals of nouns or pronouns early became obsolete, except in written works. A case occurs in Orgalaðad “Day of the Two Trees”, but since these S. nouns were all derived from Quenya names of the 6-day week, brought from Valinor, it may be due to an attempt to imitate Q. duals, such as ciriat 2 ships. In any case -d was later lost, and so we have argonath “the group of (two) noble stones” instead of *argonad (1972 letter to Richard Jeffery, Let/427).
This Sindarin dual was derived from ✶ata the “numerative dual ending”. The other ancient (and more common) dual ending was ✶-ū, but since final vowels vanished in Sindarin this u-dual vanished completely and there are only remnants of it in fossilized forms like the plural (really a dual) of lheweg “ear”:
- *slasū > lhahu > lhau, spelled lhaw “ears” (PE17/62, 77; Ety/LAS²).
There are several clear -ad duals attested in Sindarin, however:
- Orgaladhad “Day of Two Trees (galadh)” (LotR/1110).
- samarad “*two neighbors (sammar)” (VT48/20).
- nobad “thumb and index finger as a pair, (lit.) *pickers” (VT48/5).
It is conceivable that the -ad dual remained active into the First Age (when names like Orgaladhad were introduced) and only fell out of use by the Third Age.
Conceptual Development: Tolkien mentioned an archaic dual form in the Gnomish Grammar of the 1910s that used the dual suffix -wi:
Nouns only preserve in living forms singular and plural. Traces occur of the commonest old duals but these are now confined to such as: ... mabwi “(a pair of) hands” ... talwi “feet” ... hent [or] henwi “eyes” ... hethwi “brother and sister” ... hunt (old -nt ending) “the nose (originally nostrils)”. Dual -wi, -wint, -wid [nominative, genitive, dative]. Adjectives follow in singular but without mutation (GG/10-11).
Signs of both the -wi/-ui and the -(n)t dual suffixes can also be seen in declension charts appearing in Gnomish Lexicon Slips (addendums to the Gnomish Lexicon) written towards the end of the 1910s (PE13/117-118):
- engant dual of engan “spear”.
- ceralt dual of ceral “jar”.
- golui dual of gôl “gnome”.
- gui(w) dual of gweg “man”.
Some dual forms are also mentioned in the Early Noldorin Grammar of the 1920s and follow roughly the same patterns:
- badui dual of ᴱN. bad “way” (PE13/120).
- [deleted] *duilent dual of ᴱN. duilen “swallow” (PE13/120 note #4, give in mutated from nuilent).
- him dual of ᴱN. hen(n) “eye” (PE13/122, replacing deleted henu).
There is also a remnant of the -(n)t dual in The Etymologies of the 1930s: the dual hent for N. hên “eye” (EtyAC/KHEN-D-E).