In Sindarin and Noldorin an isolated short [u] was generally lowered to [o]. This sound change was discussed by Bertrand Bellet in his essay on Vowel Affection in Sindarin and Noldorin (VASN). He proposed that it was a general change inhibited by a few special circumstances (see below). One complication in analyzing this sound change is that often [u] > [o] was the result of a-affection. There are nevertheless many examples of [u] > [o] that cannot be explained this way:

• ✶nukotto/a > S. Nogoth “Dwarf” (WJ/413).
• ✶urkō > S. orch “Orc” (WJ/390).
• ᴹ√NUS > nusina > ON. nuhina > nohen > N. noen “wise, sensible” (EtyAC/NUS).
• ᴹ√RUS > ON. russe > N. rhoss “polished metal” (Ety/RUS).
• ᴹ√SUK > N. sogo “to drink”, present tense sôg (Ety/RUS).

This last example is of interest because the (Noldorin) past tense of this verb was sunc, where [u] > [o] does not occur. It is representative of one of the conditions where the sound change [u] > [o] is inhibited. These conditions were described in detail by Bertrand Bellet (VASN), and I agree with most of his conclusions (but see below).

[u] was preserved before nasals: There are numerous examples where [u] was preserved if it appeared before a nasal, either [m], [n] or [ŋ]:

• ᴹ✶krumbē > N. crum “left hand” (Ety/KURÚM).
• ᴹ√TUMPU > N. tump “hump” (Ety/TUMPU).
• S. mund “bull” vs. Q. mundo (Let/423).
• ᴹ√LUT > N. lhunt “boat” vs. ᴹQ. lunte (Ety/LUT).
• √UÑG > S. ungol [uŋgol] “spider” (Let/180; PE22/160).
• ᴹ√SUK > N. sog- “to drink” with past tense sunc [suŋk] (Ety/SUK).

Apparent exceptions to this rule can be generally be explained by a-affection:

• ✶ambuna > S. amon “hill” (PE17/93).
• ᴹ✶krumbā > N. crom “left” (Ety/KURÚM).

This exception before nasals appeared in David Salo’s Gateway to Sindarin (GS/§4.88), but Bertrand Bellet was more conservative and assumed o > u was only inhibited before nasal + stop (VASN). A possible counterexample is S. thond vs. Q. sundo “root”.

[u] was preserved before syllables with [u] or [w]: Where the syllable containing [u] was followed by another syllable containing a [u] or a [w], then the [u] was preserved in both syllables:

• ✶ngurū > S. guru “death” (PE17/87).
• ✶uruku > S. urug “bogey” (WJ/390).
• ᴹ✶tyulussē > N. tulus “poplar-tree” ( Ety/TYUL).
• ᴹ√KUR [> kurwe] > curw > N. curu “cunning” vs. ᴹQ. kurwe “craft” (Ety/KUR).

Apparent exceptions to this rule might also be explained by a-affection:

• √(U)RUÞ [> urutha > urotha] > S. oroth “rage” vs. Q. ursa (PE17/188).
• ᴹ√TULUK [> tuluka > tuloka] > N. tolog “stalwart, trusty” vs. ᴹQ. tulka “firm, strong, immovable, steadfast” (Ety/TULUK).

Bertrand Bellet further suggested that the lowering of [u] might be inhibited by a following syllable containing [i], but he provided no examples. He also noted several Noldorin counterexamples involving the infinitive suffix -ie or -i:

• ᴹ√DUL [> dulie > dolie] > dœlio > N. delio “conceal” (Ety/DUL).
• ᴹ✶or-tur- [> orthuri- > othori- > othœri-] N. ortheri “conceal” (Ety/TUR).

Although obscured by the later sound changes of [o] > [œ] > [e] by way of i-affection, these forms can only be explained by a lowering of [u] to [o]. Until a clear example emerges that supports an alternate scenario, I’m going to assume that the presence of a following syllable containing [i] did not block the lowering of [u] to [o].

Long [ū] was preserved: Where the [ū] was long it did not become [o]. This is true of original primitive [ū], and also of cases where primitive [ō] became [ū]:

• √RUTH > S. rûth “wrath” vs. Q. rúsë (PE17/188).
• ✶(s)lōkō > S. lhûg “snake” (PE17/160).
• ᴹ√THŪ > N. thûl “breath” vs. ᴹQ. súle (Ety/THŪ).
• ᴹ√RŌ > ON. rōna > N. rhûn “east” (Ety/RŌ).

[u] in diphthongs was preserved: The only primitive diphthong starting with [u] was ui, and this remained unchanged:

• ✶duinē > S. duin “river” (VT48/23).
• ✶luini > S. luin “blue” (VT48/24).
• ᴹ✶phuine > N. fuin “night” (Ety/PHUY).
• ᴹ✶tuilelindō > N. tuilinn “swallow” (Ety/TUY).

Diphthongs ending with asyllabic u̯ likewise did not become o, such as: ✶auri [au̯ri] > S. aur “day” (PE17/148).

Other aberrations: There are a few apparent exceptions to the rules above with a final [u] that can be explained by the presence of a primitive nasal [m] that later spirantalized and then vanished after [u]. In these examples, the [m] would have survived long enough to prevent [u] > [o]:

• ᴹ√ERE > N. eru “desert” vs. ᴹQ. erume (Ety/ERE), likely from primitive *✶erumē.
• ᴹ√TELU > N. telu “dome” vs. ᴹQ. telume (Ety/TEL), likely from primitive *✶telumē.

There are a few examples of where it seems an ancient initial u survived long enough to prevent [u] > [o] but was then lost, for reasons not entirely clear (hat tip to Elaran for pointing out this explanation for these words):

• √(U)RUÞ > S. ruthra- “to rage” (PE17/188).
• √(U)RUS > S. rust “copper” vs. Q. urus(t)- (VT41/10).
• √(U)RUS > S. rusc “fox” (VT41/10).

There are a couple of aberrant forms that are harder to explain:

• ✶guruk > S. gorog “horror” (WJ/415).
• ᴹ√NUT > N. nud- “to tie” vs. ᴹQ. nutin “I tie” (Ety/NUT).

In the first example, Bertrand Bellet suggested that ✶guruk might actually be the stem form and the original primitive form was *✶gurukā, so that the [u] > [o] via a-affection. The second example from The Etymologies defies any easy explanation, especially since Tolkien gave this verb as nod- elsewhere (Ety/WŌ); perhaps it is simply a mistake, as suggested by Bertrand Bellet.

Another difficult-to-explain aberration is S. thond “root” vs. Q. sundo, where it u > o despite appearing before a nasal. In The Etymologies Tolkien initially derived these words from the root ᴹ√SUD > N. sunn, ᴹQ. sundo, which he then revised to ᴹ√STUD > N. thonn, ᴹQ. sundo (Ety/STUD, EtyAC/STUD). In the Unfinished Tales, the Quenya form appears instead as sunda in Tarmasundar (UT/166). I think the likeliest explanation is that Tolkien wanted Q. sundo and S. thond but couldn’t reconcile them phonetically, vacillating between various ancient and modern forms trying to find a compatible set that he liked: *stundō vs. *stundā, N./S. thunn vs. thond, Q. sundo vs. sunda.

Conceptual Development: Isolated short u was quite common in the Gnomish of the 1910s, so it seems unlikely this general sound change was an aspect of Tolkien’s earliest conceptions of the languages. Roman Rausch suggested that [u] > [o] in final closed syllables in Gnomish in his Historical Phonology of Goldogrin (HPG/§1.5):

• G. culu “gold” vs. genitive culon, dative culor (GG/14).

There is a hint of this sound change in the Early Noldorin Grammar from the 1920s:

(2) o (from ŏ or ŭ) mutates to ui, u (unaccented occasionally > wi) (PE13/122).

However, this sound change is not entirely consistent with the attested examples in the early Noldorin Word-lists and Noldorin Dictionary, for example: ᴱN. grug “angry, grieved” (PE13/145) or ᴱN. lhub “fat” (PE13/149). Roman Rausch suggested in his Historical Phonologies of Ilkorin, Telerin and Noldorin around 1923 that in fact sometimes in Early Noldorin the opposite sound change of [o] > [u] occurred before nasals (HPITN/§4.2.2):

• ᴱN. gunn “dragon” vs. ᴱQ. kondo (PE13/162).

This is also supported by a note in the Early Noldorin Grammar:

(4) u (from ŏ + nasal, ŭ) mutates to ui (PE13/122).

GS/§4.88 WGHC/§66i WGHC/§73i possible aberration: thonn (after short final vowels vanished?) hû @@@

HPG/§1.5