S. [œi] became [ui] or [y]; [œi] > [ui|y]
One open question is how the diphthong [œi] developed in Sindarin, assuming it occurred at all. This archaic diphthong is relatively common in the Noldorin of the 1930s (where it became [ei]), arising in the phonetic history of plural forms of some words with a base vowel o, where the raising of [o] to [u] was often inhibited in the final syllable of polysyllables:
However, [œi] was not a factor in the phonetic development of Sindarin plurals, because the o was generally raised to u and thereafter fronted to y.
Nonetheless, the diphthong [œi] could theoretically arise in Sindarin in cases where the raising of o did not occur but fronting to œ still happened. This might occur in words with primitive forms like -uCya, -oCya or -oCye, since the non-final y/ı̯ would not cause raising but would still cause fronting. If this were to occur, these would develop through -oCı̯(ǝ) > -œCı̯ > -œiC, though the case of -uCya first involves a-affection of u > o. There are a few Sindarin words that seem to have these primitive forms, but they show varying vowels in the resulting Sindarin words:
For the last example, compare its development to the Noldorin word from The Etymologies: N. feir < fœir vs. ᴹQ. forya “right (hand)”, both derived from the root ᴹ√PHOR (Ety/PHOR). Based on examples like fuir, Roman Rausch suggested one possible phonetic development of [œi] was to [ui] in his article On the Diphthongs ei, ai in Noldorin and Sindarin (DEANS/§4.5, 2008); David Salo made a similar suggestion in Gateway to Sindarin (GS/§4.188, 2004). However, Bertrand Bellet pointed out that ruin and fuir might have instead developed from *rūnya and *phōryā > *phūryā, and so didn’t involve [œi] at all (Vowel Affection in Sindarin and Noldorin, VASN, 2005). Elsewhere another Quenya cognate of S. ruin is given as Q. rúnya “red flame” (SA/ruin), which supports but does not confirm Bellet’s position.
Bellet didn’t suggest a Sindarin development for [œi]. However, based on the example ✶oronyē [> *œrœni(e) > *œrœin] > eryn (PE17/119), a second possibility is that [œi] > [y] might be the normal Sindarin sound change; this example was published after Bellet’s article. There is a Noldorin example that might be a precursor to this phonetic development: ᴹ✶ronyō > N. rhŷn “chaser, hound of chase” (Ety/ROY¹). But eryn also has an alternate etymology as an archaic plural form of S. orn “tree” from primitive oronī (PE17/33), making this phonetic development ambiguous as well.
Also of interest is that these (hypothetical) developments of [œi] match the distribution of Sindarin and late Noldorin plurals of polysyllabic vs. monosyllabic words which end in a single consonant and whose final syllable contains o. Compare:
Given this alignment, it is possible that final combinations like -oCya or -oCye underwent i-raising for o to u despite the fact that the [j] or [ı̯] was not final at the time the raising would normally have occurred. Either way, it seems the normal Sindarin result of ancient -oCya or -oCye is -uiC (monosyllables) or -yC (polysyllables), either -oCı̯(ǝ) > -œCı̯ > -œiC > -uiC/-yC or -oCı̯(ǝ) > -uCı̯(ǝ) > -yCı̯ > -uiC/-yC.
Note that Roman Rausch identified an alternate development, based on a varying list of plurals for S. og(o)l “evil”: egl, eigl, eigil (PE17/149). The middle plural hints that perhaps [œi] > [ei] as in Noldorin, and Rausch suggests this might sometimes have occurred in Sindarin as well (DEANS/§4.5). However, the numerous plural forms make it clear Tolkien was vacillating among several possible outcomes. I hesitate to draw any conclusions from this particular example, especially since none of these plurals resemble normal Sindarin plural patterns. See the entry on og(o)l for a discussion of these plurals.
Phonetic Rule Elements
Phonetic Rule Examples
|œrœin > œryn||œi > y||✶oronyē > S. eryn||✧ PE17/119|