The earliest iteration of this root was ᴱ√ẎATA “join” in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. yarta “yoke” and ᴱQ. yatta “neck; isthmus” (QL/105). In the contemporaneous Gnomish Lexicon it had derivatives like G. gada- “join, connect, unite” and G. gath⁽⁾ “neck” < ᴱ✶yatt- (GL/36). It reappeared as ᴹ√YAT “join” in The Etymologies of the 1930s with derivatives like ᴹQ. yanta/N. iant “yoke” and ᴹQ. yanwe/N. ianw “bridge” < ᴹ✶yatmā (Ety/YAT). Above ᴹ√YAT there was a deleted root ᴹ√YATH with a single derivative ᴹQ. yatta “neck, isthmus” that was not deleted (EtyAC/YAK). ᴹQ. yatta may have been transferred to ᴹ√YAT, though as pointed out by Carl Hostetter and Patrick Wynne it might also have been transferred to ᴹ√YAK, which included among its derivatives ᴹQ. yat (yaht-) “neck” (Ety/YAK).
In later writings, Q. yanta and S. iant were used for “bridge” (LotR/1123; SD/129). The word Q. yanwë reappeared with the gloss “joining” in notes from 1969, initially derived from ✶yadme but this was deleted and replaced by √YAN (VT49/45-56). Carl Hostetter suggested that rejected ✶yadme probably reflected the earlier phonological developments seen in The Etymologies whereby *tm* became *dm*, then *nm*, and ultimately *nw*. However, sometime in the late 1950s or early 1960s Tolkien abandoned tm > dm > nm > nw, deciding instead that *tm* > *tw*, which may have motivated him to change the root ᴹ√YAT to √YAN. A similar revision can be seen in change of ᴹ√TEK to √TEÑ in the derivation of Q. tengwa “letter”.
Neo-Eldarin: For purposes of Neo-Eldarin, I prefer to retain the phonological developments of the 1930s to 1950s whereby km, tm > gm, dm > ñm, nm > ngw, nw. As such, I think it is better to retain the 1930s form of the root ᴹ√YAT “join”, which also helps avoid conflict with √YAN “wide, extensive, large”. As the above discussion indicates, Tolkien seems to have been vacillating on tm > tw vs. tm > nw as late as 1969.
A root Tolkien introduced in the late 1950s to serve as the basis for various “land” suffixes in Sindarin. The most detailed explanation appeared in a set of documents labeled “Changes affecting Silmarillion nomenclature”:
> The endings -ion, -ien, -ian(d) in place names. These have various origins. In Sindarin -ion is usually from -ı̯aun. This in origin is from yānā, √YANA-, extension of yā- (cf. YAGA, gap) “wide, large, extensive”. S iaun “roomy, wide, extensive” ... -iand (-ian) is from yandē “a wide region, or country” ... This was often used in plural of a single country (especially if it contained a varied topographical apsect) > iend, ien (PE17/42).
Thus all three suffixes -ian(d), -ien(d), -ion originate in the root √YAN. The suffix -ian(d) “land” (Beleriand) is the simplest, just a reduction of ancient -yandē. The suffix -ien(d) “lands” (Anórien) is a plural variant of -ian(d). The suffix -ion (Eregion) is S. iaun “wide, extension” used as a suffix (so perhaps = “✱extent”), becoming -ion because of the usual sound change whereby *au* become *o* in polysyllables.
In this same document, Tolkien also considered introducing a root √YŎNO “wide, extensive”, going so far as (temporarily) rejected the very well established word Q. yondo “son”. This √YON was blended with √YOD “fence, enclose”, and served as the basis for the suffix -ion, but Tolkien ultimately marked these notes with an “X” to reject them, perhaps because they only explained the suffix -ion, whereas √YAN could explain all three suffixes.
√YAN was mentioned in passing in other documents from this period, variously glossed “vast, huge” (PE17/99), “wide” (PE17/115), and “extend” (PE17/155), and in one place given a variant √YAD (PE17/115). In notes from December 1959 (D59) √YAN was contrasted with √ƷAN, the former meaning “wide” and the latter meaning “long”, both with the basic sense “extend” (PE17/115); see the entry on √HAN for further discussion.