In Primitive Elvish, nasals assimilated to following stops and aspirates. It seems this change occurred first for the velar nasal [ŋ], somewhat later for the dental nasal [n] and may have occurred for the labial nasal [m] only after the Common Eldarin period, though the assimilation of [m] does seem to occur in a very ancient period for all the Eldarin child languages (Quenya, Sindarin and Telerin).

The primitive assimilation of nasals was an idea that dates back to Tolkien’s earliest conception of the Elvish languages. Tolkien mentioned this assimilation in the Qenya Phonology from the 1910s:

There certainly existed a nasal in all positions but {it is not >>} most are not found “free” in a vocalic state whereas prehistorically all nasals were assimilated to the timbre and articul[atory] position of the succeeding consonsant whether semi-vocalic or vocalic (PE12/11).

Tolkien mentioned these assimilations in both versions of Tengwesta Qenderinwa, from the 1930s (TQ1) and the 1950s (TQ2):

[ŋ] tended very early to become denasalized to [ʒ]. But before this tendency operated it became assimilated to following stops: thus suffixal ŋt, ŋth, ŋd > nt, nth, nd ... it was assimilated to [m] before p, ph, b; but ŋv probably became ŋgw though no certain cases are known. ... [n] became assimilated also to all stops (p, ph, b, k, kh, g), but originally remained before continuants y, w, {(v)}, m. ... [m] originally remained unchanged in contact, though examples of m + k, kh, g are rare, and of m + ŋ, ʒ there are no examples (TQ1, PE18/54).

Nasal consonants tended to be assimilated to the place of articulation of the following consonant — if a stop or [s]. The nasal [ŋ] became n before t, th, d, s; m before p, ph, b ... The nasal [n] became ŋ̃ before k, kh, g, ʒ (which did not occur as suffixal combinations but only in talta-forms of ³√TALAT-bases) ... n was also assimilated to m, before p, ph, b (non-suffixal), but not before m in suffixal combinations. The nasal [m] was originally not assimilated to following consonants, though it was early assimilated to [ŋ] before k, kh, g in talta-forms (TQ2, PE18/102).

Tolkien also discussed these assimilations in the Outline of Phonetic Development from the 1930s (OP1) and from the Outline of Phonology from the 1950s (OP2):

In CE [ñ] became [ʒ] (originally probably via nasalization of the neighbouring vowels, the nasality of the whole group being then later abandoned), except where it had become assimilated to some other nasal: as ñt > nt ... m, n as a rule remained unchanged [in CE] ... The nasals were frequently assimilated to dentals: [ñ] always; m always except in mn which was preserved. m, ñ + t, d > nt, nd (OP1, PE19/46-47).

Before stops and aspirates. Assimilation [of nasals] was regularly carried out and maintained in Quenya. The oldest assimilation was probably that of ñ to labials and dentals; ñ thus becoming m or n in CE was preserved as a nasal. But also of CE age was the assimilation of dental n to m before labials and ñ before k, kh, g. The assimilation of m was probably not accomplished in CE. It did not occur in Telerin, where e.g. mt > md, mpt (OP2, PE19/102).

According to editor’s note #144 on PE19/102, the last Telerin note was added in the margin in green pen and probably dates to around 1970, and might be better interpreted as mt/md > mpt. This is the only text I’ve found that indicates nasal assimilations might not have occurred in all Eldarin languages, and it seems to be a very late (and possibly transient) idea.

The quotes above indicate Tolkien was not entirely certain about the timing of these changes; he seems consistent in that ŋ-assimilation occurred first followed by [n] and then [m], but while ŋ-assimilation must have occurred in CE (otherwise ŋ would have become ʒ), m-assimilation properly belongs to the history of the child languages (and he may sometimes have imagined this was true of n-assimilation as well). The exact timing of the changes may have depended on whether the combinations were the result of suffixion or from the vowel variations that produced TALTA forms.

Nasal assimilations remained an active sound change in Quenya probably for its entire history up through modern Quenya, given the extreme constraints on Quenya phonotactics. For example, it still occurred after Quenya-only sound changes like the Quenya syncope and stop/nasal metathesis, for example:

• ✶lepene > lepne > Q. lempe “five” (VT42/24, VT47/10).
• ✶min(i)kewe > minikwe > Q. minque [miŋkwe] “eleven” (VT48/7-8).

In cases like [n] > [ŋ] before k, g, the sound change was obscured because Tolkien generally wrote nk (nc), ng for [ŋk], [ŋg], since this was phonetically unambiguous. In Sindarin, however, at least the assimilation of labial [m] before by a dentals ceased to be an active sound change at some point in the language’s history, since combinations like md were possible in late compounds: S. amdir “hope” (MR/320); S. Ramdal “Wall’s End” (S/122), N. Rhamdal (Ety/TAL, LR/262). Examples like S. penbed “not pronouncable” and [deleted] S. danbeth “answer” hint that this may have been true of [n] + labial combinations as well, but there the evidence is more ambiguous. There are definitely cases where such assimilations occur in Old Sindarin/Old Noldorin, however:

• √GLIM > S. glintha- “glance” (WJ/337).
• √KEM + √TAN¹ (> *centhan) > N. cennan “potter” (Ety/TAN; EtyAC/KEM).
• √elen-barathī > el-mbereth > S. Elbereth “Star-queen” (MR/387, PE17/22).

Neo-Eldarin: For purposes of Neo-Eldarin, it is easiest to model this phonetic development as occurring in Primitive Elvish. This has barely any effect on the child languages if one assumes it remained an active sound change up through Tarquesta and (at least) Old Sindarin.

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