OS. stressed [wŏ] became [wa]; [wŏ́] > [wa]
In the history of Sindarin development, when short and stressed the primitive prefix wo- “together” became wa-, and eventually developed into gwa- after initial [w] became [gw]. When unstressed, this prefix did not change its vowel and developed into go-, because later [wo] became [o]. It seems likely Tolkien introduced the sound change wŏ́ > wa primarily to explain the variable form of this prefix gwa-/go-: this variation existed all the way back in Gnomish, and persisted through all conceptual stages of this language. A similar sound change occurred in Welsh (WGCH/§65v).
In Primitive Eldarin the combination wo was fairly rare, so there aren’t any clear examples of this phonetic change other than the one Sindarin prefix noted above. However, Tolkien did explicitly describe this change a number of times:
- The form wŏ if stressed > wa in Eldarin (Etymologies, 1930s, Ety/WŌ).
- When stressed the sequence wo was usually changed to wa [in Quenya]. A similar change occurs in Noldorin. (Outline of Phonetic Development [OP1], 1930s, PE19/53).
- When stressed the sequence woi > wai, parallel to the change of wo > wa [in Quenya]. (Outline of Phonology [OP2], circa 1950, PE19/106).
- gwa- is regularly developed from *wo > *wa > gwa, when stressed in prehistoric Sindarin. (Quendi and Eldar, circa 1960, WJ/367).
The first three notes indicate this change occurred in Common Eldarin, and applied to both Quenya and Sindarin. The last hints that Tolkien might have changed his mind around 1960 and made this a change in Prehistoric Sindarin only; later in the same paragraph he attributed another change to the PQ [Primitive Quenderin] period, so it is possible that his use of “prehistoric Sindarin” in this phrase is significant. David Salo also attributed this sound change to the Old Sindarin period (GS/§4.77).
One further possible example (the only one not involving gwa-/go- “together”) is:
Since we have final -wō > -wa in both Quenya and Noldorin, this might be an example of this phonetic rule. In this conceptual period, however, the change applied to primitive short stressed wŏ, and this example is long wō (and quite possibly unstressed). It is conceivable the final wō shortened first, then became wa, but the evidence is that long final vowels shortened much later in Quenya than in Noldorin/Sindarin, such as the persistence of long final vowels in the accusative into Classical Quenya. This makes it less likely this was the same sound change: David Salo theorized this was a distinct phonetical development, where final -wō > -wā in the Common Eldarin period (GS/§4.15).
As a final note, Tolkien mentioned that later gwa- became gwo- and then go- after stress shifted in Sindarin (WJ/368), indicating that wŏ́ > wa (or at least gwa- > gwo-) remained an active phonetic rule for some time. David Salo also recorded this later application of the phonetic change (GS/§4.103).
Conceptual Development: As noted above, the gwa-/go- variation dates all the way back to Gnomish, but as suggested in Roman Rausch’s Historical Phonology of Goldogrin (HPG/§1.4), in the earliest period this change seems to be reversed, in that unstressed wa > wo rather than stressed wo > wa:
[G.] go unaccented form < gwa-. [primitive] *ŋu̯a. cp. Q. ma- together, in one, etc. (GL/40)
This also seems to be true of the Early Noldorin of the 1920s:
[ᴱN.] go-. Originally unaccented form < gwá- = together, etc. (Q, T va-) (PE13/162)
Based on the Early Quenya and Early Telerin forms ma- and va-, this change to unstressed wa seems to apply only in Gnomish/Early Noldorin, not to the other languages.
By the 1930s, Tolkien revised the sound change to stressed wŏ́ > wa, and designated it as a Common Eldarin change that applied to both Quenya and Noldorin (Ety/WŌ, PE19/106). Base on the above notes in the Outline of Phonology [OP2], this seems to still be the case around 1950 (PE19/106). As suggested above, around 1960 Tolkien may have changed his mind and made this a Sindarin-only change again (WJ/367) but the evidence is not conclusive.
Neo-Eldarin: Despite the inconclusive evidence, I prefer to assume that this sound change is Sindarin-only for the purposes of Neo-Eldarin, in part because it helps justify the retention of Early Quenya forms beginning with quo-. See the discussion in the entry for the Quenya Sound change: initial [wo] became [o].