In (Old) Sindarin the ancient primitive diphthong [eu] merged with [iu], and later [iu] became [ȳ]. The clearest representation of this change is in a chart describing the phonetic development of Primitive Elvish diphthongs in all three languages (Quenya, Telerin and Sindarin), appearing in notes associated with Tolkien’s discussion of Hands, Fingers and Numerals from the late 1960s (VT48/7). Here Tolkien made it clear that [eu] > [iu] first, and then later [iu] > [ȳ]. In The Lord of the Rings Appendix E, Tolkien also mentioned that primitive diphthongs [eu] and [iu] both became [y], but without describing the intermediate changes (LotR/1115).

It’s easy to find examples of [eu] > [ȳ] in Sindarin, but the intermediate steps need to be reconstructed:

• ✶deulē [> *diule] > S. dýl “mistake” (PE17/151).
• ✶keu-rā [> *kiura] > S. cŷr “renewed” (VT48/7-8).
• √LEWEK [> *leukā > *liuka] > S. lŷg “snake” (PE17/160, LotR/1115).

A similar change took place in Welsh, where iu and ēu (long diphthong) first blended to iu and then eventually became y or i (WGHC/§76ix). The Primitive Indo-European short diphthong ĕu, however, became ou in Welsh’s phonetic history, with various later developments depending on context (WGHC/§76i).

Conceptual Development: The blending of [eu] and [iu] was an old idea of Tolkien’s, dating all the way back to Gnomish, but Tolkien changed his mind several times on the ultimate end-product of this sound change. In an early chart of Gnomish vocalic developments, Tolkien indicated that [eu] ultimately became [iu] (iw) when stressed, but both [eu] and [iu] became [io] or [jo] (yo) when unstressed (PE15/13). As pointed out by Roman Rausch (HPG/§1.2), several examples in the Gnomish Lexicon show both phonetic developments, perhaps the result of varying primitive stress patterns, while others examples show only io:

• ᴱ✶n’reu̯ > G. drio “hero” beside driw (GL/30).
• ᴱ√giu̯i > G. giol “pregnant” beside giwl and giwol (GL/39).
• ᴱ√giu̯i > G. giothra- “to germinate, be conceived” (GL/39).

In a chart of Early Noldorin diphthongs from the 1920s, Tolkien seems to have dropped the survival of stressed [iu], and so that [eu], [iu] became [io] in all cases (PE15/64). There are, however, no clear examples of this sound change in the 1920s.

By the 1930s, the normal Noldorin phonetic development seems to have been for both [eu] and [iu] > [ū]:

• ᴱ✶ndeuro > N. Dûr “Successor” (Ety/NDEW).
• ᴱ✶libda [> *(g)liuda> *glūda] > N. glúð “soap” (Ety/LIB²).
• ᴱ✶tiu̯kā > ON. tūka > N. tûg “thick, fat” (Ety/TIW).

There are a couple Noldorin examples that still show [eu] > [io], with varying ultimate effects:

• ᴱ✶beu̯rō > biuro > ON. bioro > N. beor “follower, vassal” (Ety/BEW).
• ᴹ√SNEW [> *sneuma] > ON. sniuma/snȳma > N. hniof/hnuif “noose, snare” (Ety/SNEW).

In the second example, the Noldorin forms hniof/hnuif replaced a deleted form nû < ᴹ√SNEW which shows the more normal Noldorin phonetic development of [ū] < [eu]. Christopher Tolkien describes this revision: “The N. forms were changed to ON. sniuma and snýma; hniof (plural hnyf) and hnuif.” This may represent one long complex development of [iu] > [ý] > [io] > [ui]. However, I think it is more likely to represent two parallel developments:

• ᴹ√SNEW [> *sneuma] > ON. sniuma > N. hniof
• ᴹ√SNEW [> *sneuma] > ON. snȳma > N. hnuif

There are examples elsewhere of interchange of [y] and [ui] in Noldorin, such as the plural of N. amon “hill”: emuin, emyn (Ety/AM²), though with the plural of amon it seems [ui] > [y]. Assuming my “two parallel developments” theory is correct, the first development to [io] seems analogous to beu̯rō > biuro > bioro > beor (Ety/BEW), and the second development might be a precursor to the Sindarin development to [ȳ].

As these examples of [eu] > [io] demonstrate, it seems that first [eu] > [iu], and then underwent further phonetic developments, as was also the case in Sindarin. It is plausible that first [eu] > [iu] at all stages of the language’s conceptual development; it seems Tolkien borrowed this behavior from Welsh early on, and retained it thereafter.

It is not clear when Tolkien shifted from the Noldorin phonetic rule of [eu]/[iu] > [ū]/[io] to the Sindarin phonetic rule of [eu]/[iu] > [ȳ], but by the late 1950s it seems the new Sindarin paradigm was well established, when it was canonized by its publication in The Lord of the Rings.

GS/§4.28 GS/§4.29 GS/§4.60 WGHC/§76i [PIE eu > Brit ou > W u] §76xi [PIE iu, ēu > Kelt iu > W iw or yw] @@@,

Exceptions:

• beu̯rō > biuro > bioro > bior > Bëor (Ety/BEW)
• sneuma > sniuma > snȳma > hniof > hnuif (Ety/SNEW)

See OS. [eu] became [iu] for discussion.

See OS. [eu] became [iu] for discussion.