There is evidence that in both Sindarin and Noldorin, the labial nasal [m] became [w] after another nasal, [n] or [ŋ] (the latter only in Sindarin, see below). This sound change is mentioned in some notes from the late 1960s on the phonetic developments of combinations of nasals:

[S] nm > nw̃ > n. (PE22/148).
S nm > nw ... ŋm > ŋw̃ > ¯w (PE22/149).

The clearest examples of this change are the etymologies of N. tinw and (some of) the etymologies of S. têw or tîw:

• ᴹ✶tinmē > N. tinw “spark, small star” (Ety/TIN).
• ✶tekmā > tegmā > teŋw > S. tēw “letter” (PE17/43).
• ✶teŋmā > teŋwā > tēwa > S. tîw (PE22/149).

Note that this sound change may simply be part of the general trend whereby non-initial [m] usually became [v] (and then to -w when final).

Conceptual Development: The sound change [nm] > [nw] was more applicable in Noldorin than Sindarin, since in (Old) Noldorin all voiced stops became nasals before nasals, so that the combination nm could also arise from both dm and tm (the latter because voiceless stops were voiced before nasals). For example:

• ᴹ✶yatmā [> *yadmē > *yanme] > N. ianw “bridge” (Ety/YAT).

Remnants of these Noldorin sound changes for stops + nasals can be seen in the Sindarin development of tekmā > tegmā > teŋw given above, which shows [km] > [gm] > [ŋm]; an example on the next page has tekma > teŋ̃ma as well (PE17/44). However, in notes associated with Elvish Numerals written in the late 1960s, Tolkien revised this phonetic development in Sindarin, so that the nasalization of stops before nasals was limited to homorganic stops and only [bm], [dn] became [mm], [nn] (VT42/26). This forced Tolkien to revise the root for S. têw “letter” (√TEK >> √TEÑ), but [ŋm] > [ŋw] remained a factor in its phonetic development (PE22/149).

The sound change [nm] > [nw] must have been very limited in Sindarin. After the revision on the development of stops + nasals noted above, nm could only arise from primitive nm and n + m in compounds. However, examples elsewhere indicate that [n] assimilated to following labial at morpheme boundaries, as with OS. lenn-mbass > S. lembas (PM/404) or ✶n[d]an-men > [*dammen-] > S. damen- (PE17/166). Thus [nm] > [nw] could only occur from primitive nm, and we have no explicit examples of this sound change in Sindarin.

Conversely, the sound change [ŋm] > [ŋw] probably did not occur in Noldorin, since earlier still [ŋ] vocalized before [m] in Noldorin:

• ᴹ√DOƷ/DÔ > dogme > dongme [doŋme] > ON. doume > N. daw “night-time, gloom” (Ety/DOƷ).