Quenya 

-ttë

they

-ttë (1) "they", dual 3rd person pronominal ending ("the two of them") (VT49:51), replacing (also within the legendarium) the older ending -stë (which was later used for the second person only). This older ending -stë corresponds to a possessive ending -sta "their" (VT49:16), but this was presumably likewise altered to *-tta as the new ending for dual "their" = "of the two of them".