-nten ending for dual instrumental (Plotz)
-nenmost nouns have an instrumental in -nen
-nen instrumental ending (pl. -inen, dual -nten, partitive pl. -línen). Attested in ambartanen, lírinen, lintieryanen, súrinen, parmanen; see ambar (#2), lírë, lintië, súrë, parma. Tolkien noted that "most nouns have an instrumental in -nen" (PE17:62), a wording suggesting that the form of the ending may vary; given the normal development ln > ld, it is possible that it would appear as *-den when added to a noun in -l (*macilden "with a sword").
attat2 fathers or neighbours
-t (1) dual ending, on nouns denoting a _pair of something: attat "2 fathers or neighbours" (VT48:19; see _atto), máryat "her (pair of) hands" (Nam), siryat "two rivers" (VT47:11), ciriat "2 ships" (Letters:427 read ciryat as in the Plotz Letter?), maquat "group of ten" (from maqua, meaning among other things "group of five") (VT47:7), nápat "thumb and index as a pair" (VT48:5), also compare met "us two" as the dual form of me "us" (Nam, VT47:11). Other dual endings known from the Plotz letter: genitive -to, possessive -twa, dative -nt, locative -tsë, allative -nta, ablative -lto, instrumental -nten, plus -tes as a possible short locative. It may be that these endings only apply to nouns that would have nominative dual forms in -t, and that nouns preferring the alternative dual ending -u would simply add the otherwise "singular" case endings to this vowel, e.g. *Alduo rather than ?Alduto as the genitive form of "Two Trees" (Aldu). The ending -t is also used as a verbal inflection, corresponding to pl. -r (elen atta siluvat**, "two stars shall shine", VT49:45; the verb carit** "do" would also be used with a dual subject, VT49:16; cf. also the endings listed in VT49:48, 50).