A conjunction meaning “both ... and” in notes from around 1964 used to join two words (but not two sentences), as seen in the phrase: sanome tarne Olórin, Aracorno, Eomer, Imrahil, mi míse, mi telepta yo morna, mi laiqua yo ninquë, mi luinë ... “There stood Gandalf, Aragorn, Eomer and Imrahil in grey, in silver and black, in green and white, [and] in blue” (PE17/71). In the same document Tolkien described this conjunction in some detail:
> yo, a reduced form of yŭ < yū “both”: this was often used between two items (of any part of speech) that were by nature or custom closely associated, but were not “pairs” (as e.g. were hands, feet, eyes, etc.). These might be names of persons: as Beren and Luthien, Manwe yo Varda; or of things as sword and sheath, bow and arrows; or of groups as Elves and Men (Eldar yo Firimar); or adjectives, as wine white and red; long and thin; or adverbs, as far and away; also verbs, especially those with related meanings, as hear and obey, see and consider.
Tolkien further specified that yo was not used in repetitions for emphasis (“through and through”) or synonyms like alternative names. But it could be used for words that were contrasting opposites such as “head and foot, top and bottom, up and down, young and old, men and women”.
Conceptual Development: In the Qenya Lexicon of the 1910s ᴱQ. ya(n) was the usual word for “and” (QL/104), but by the end of the 1920s this was replaced by ar “and” which Tolkien mostly used thereafter. However, in a number documents from the late 1940s Tolkien used ye (PE23/91-92, 95), yu (PE22/125; PE23/110), or yo (PE23/97) for “and”, with yo “and” also appearing in drafts of The Lord of the Rings (SD/56). By the 1950s ar “and” was restored, but Tolkien seems to have retained yo in a more limited dual sense, as described above.
yo conj. "and", "often used between _two _items (of any part of speech) that were by nature or custom clearly associated, like the names of spouses (Manwë yo Varda), or "sword and sheath" (*macil yo vainë*), "bow and arrows" (quinga yo pilindi), or groups like "Elves and Men" (Eldar yo Fírimor but contrast eldain a fírimoin [dative forms] in FS, where Tolkien joins the words with a, seemingly simply a variant of the common conjunction ar). In one source, yo is apparently a preposition "with" (yo hildinyar* = "with my heirs", SD:56).