In Sindarin and Noldorin, any final [v] or [w] after the vowel [u] vanished, as noted by David Salo (GS/§4.173, GS/§4.176, GS/§4.177). The likely mechanism was [v] > [w], and then [w] became [u] which blended with the preceding vowel. This sound change also occurred after the diphthong au, as noted by Tolkien himself:

The adjective *nāba > nǭv, nǭf only remained current in the Northern dialect, where the name Novrod originated. In the other dialects nǭv, as a stressed independent word, proceeded to nauv > naw (with usual loss of final v after au, u [emphasis added]), and this word ceased to be used in current speech (Quendi and Eldar, circa 1960, WJ/414).

There are numerous examples:

• ✶Orǭmē > Oraúmh > Araúv > S. Áraw (PE17/153), a similar etymology appears on WJ/400.
• ✶nābā > nauv > S. naw “hollow” (WJ/414).
• ✶srāwe [> *rhauw] > S. rhaw “flesh” (MR/350).
• ᴹ√TIN + ᴹ√DOM > tindumh > tindu > N. tinnu “twilight” (Ety/DOMO, TIN).
• ᴹ√SAB > ON. sōba [sǭba > *sauv] > N. saw “juice” (Ety/SAB).
• ᴹ✶dōmi > dūmh > N. dū “night” (SD/302).

In Noldorin the same disappearance occurred after the diphthong [ui]:

• ᴹ✶tuimā > tuiw > N. tui “sprout, bud” (Ety/TUY).
• ᴹ√ROY¹ > N. rhui(w) “hunt, hunting” (Ety/ROY¹).
• ᴹ✶et-kuiwē > N. echui(w) “awakening” (Ety/KUY).

The last example reappears in Sindarin: S. Nen Echui “Water of Awakening” (SA/cuivië) and S. echuir “spring” (LotR/1107), so perhaps the same reduction after [ui] occurred in Sindarin as well.

There are a number of Sindarin and Noldorin examples that indicate that [v] did not vanish after [u] medially, consistent with Tolkien’s statement above that this was a word-final change:

• S. and N. Tinúviel “Daughter of Twilight” (S/165, Ety/TIN).
• S. miruvor (LotR/290).
• N. Uvanwaith (TI/281).
• N. dúven “*western” (Ety/NDŪ).

In Noldorin at least, there are several examples where the medial disappearance of [v] after [u] seems to take place:

• ᴹ✶ū́banō > N. úan “monster” (Ety/BAN).
• √DOM + √LIN² > N. dúlin(n) “nightingale” (Ety/DOƷ, LIN², TIN).
• ᴹ✶dōmilindē [> *dūvilin(d)] > N. duilin “nightingale” (SD/302).

The second example could be a late or reformed compound as a combination of N. dû + lhinn. However, in the third example it is difficult to explain the diphthong [ui] without assuming the [m] vanishing between long [ū] and [i], and the most plausible explanation for such as disappearance would be [m] > [v] and then it vanishing after [ū]. Since there are also Noldorin examples of medial [v] surviving after [u] in similar conditions, it is hard to say what Tolkien’s intent might have been here.

There are similar vacillations around the disappearance of medial [w] after [u]. There is S. and N. dúath < du(w)ath = dû + gwath (PE17/87, 152; Ety/DOƷ), but there are also S. Deldúwath, S. Drúwaith and S. Eluwaith (S/155, UT/385, WJ/378).

One interesting example is ✶Oromē > Orom̆ > Orow̯ > S. Araw (PE17/99), which hints that final [v] became [u] after [o] as well. There is a similar example in Noldorin, where ᴹ√KUM > cofn [> *coun] > N. caun “empty, void” (Ety/KUM). However, there are counterexamples to both of these, such as ᴹ✶tubnā [> *tomn] > N. tofn and ✶gōr(i)kubā [> *gūrigov] > S. gurgof, so this could also represent vacillation on Tolkien’s part (hat tip to Lokyt for pointing out these counterexamples).

Conceptual Developments: There are a fair number of examples of medial uv in the Gnomish and Early Noldorin of the 1910s, but it is hard to say what the phonetic developments were at the end of words given the lack of attested primitive forms in the earliest conceptual stages of the languages. This is especially true since [m] did not mutate to [v] after vowels in Gnomish, meaning there were fewer possible examples of this sound change.

Tolkien did indicate that v [ƀ] vanished after u in Early Noldorin of the 1920s, but the change was not universal. In a marginal note to the Early Noldorin Grammar of the 1920s, Tolkien said “b > ƀ; ƀ vanishes finally after ū̆ (often ī̆), medially between u - i, u - u” (PE13/132 note #109). This note was in reference to the verb ᴱN. lhui “to wash” from primitive *lub- (PE13/132), which had v [ƀ] vanishing finally in neuter singular lhú but preserved medially in masculine and feminine lhuveg and lhufes (these masc/fem forms were marked archaic and with modern form lhueg and lhues, possibly via a reformation). The verb also had neuter/masc/fem plural forms lhuir/lhuig/lhuis, an example of v [ƀ] loss between u and i.

Another clear example are the verbs ᴱN. nuv- “sink, set” and ᴱN. dadnuv- “*sink down”, with neuter 3rd. sg. ný or dadnú versus masculine (dad)nuveg (PE13/151, 164). It also had a verbal noun ᴱN. nuin “sinking, going down” from primitive ᴱ✶nubhin (PE13/164), indicating v loss between u and i. In one place Tolkien said the verb had a primitive stem form of numh- (PE13/161); compare earlier G. nûmin “the west, sinking” from the 1910s (GL/61), before Tolkien decided m > v. This indicates that the Early Noldorin sound change applied to v from both post-vocalic m and b.

GS/§4.173 GS/§4.176 GS/§4.177 WGHC/§110iii-1 [-awf > -aw] @@@