In (Old) Sindarin and (Old) Noldorin, voiced stops became nasals before another nasal. In Sindarin, this change was restricted to homorganic stops, that is [bm] > [mm] and [dn] > [nn]. In Noldorin, the change was more general (see below). For Sindarin, Tolkien described this change in notes on Elvish numerals from the late 1960s (VT42/26):

In Sindarin voiceless stops (i.e. p, t, k) before nasals became voiced > b, d, g, and then together with the original voiced stops in this position became nasals before homorganic nasals (tn, dn > nn; pm, bm > mm), but before other nasals became spirants as generally medially (pn, bn > vn; tm, dm > ðm, later ðv, ðw; kn, gn > gn > in; km, gm > gm > im > iv, iw).

Similar developments are indicated in notes from around 1969, nasalization before a homorganic nasal but otherwise retention of the voiced stop with other later developments:

dm > v. gm > u̯w ... bm > mm > m. nm > nw > n. tm > dm > df. km > gf. (PE22/148).

As indicated by this note, voiced stops in non-homorganic combinations were not nasalized, but instead underwent the normal development after vowels into voiced spirants. Although this phonetic rule was described by Tolkien, there are not any published Sindarin words which provide obvious examples of this sound change.

Conceptual Development: Determining medial developments in Gnomish and Early Noldorin is difficult given the lack of examples of primitive forms, but by the Noldorin of the 1930s it is clear this sound change was in effect. In notes on the Noldorin usage of the Feanorian Alphabet from the 1930s, Tolkien ascribed this sound change to the Old Noldorin period:

The PQ combinations ȵm, ȵn (from ȵ, ʒ + m, n) appear to have given gm, gn, falling together with gm, gn from PQ k, g + m, n. Hence ON standard spelling xy x6. Later here x g became [ŋ], as bm, bn > mm, mn, dm, dn > nm, nn; but ON having no special sign for [ŋ] retained the gm, gn spelling. The nasality is indicated by the occasional spelling by b6 = ngm, ngn (PE22/26).

Unlike Sindarin, the nasalization of voiced stops occurred in all combinations. There are a number of fairly clear examples of this sound change from The Etymologies:

• [bn] > [mn]: ᴹ✶stabnē > ON. sthamne > N. thafn “post, wooden pillar” (Ety/STAB).
• [dm] > [nm]: ᴹ✶yatmē [> yadme] > ON. yanme > N. ianw “bridge” (Ety/ƷEL, YAT).
• [dn] > [nn]: ᴹ✶patnā [> padna] > N. pann “wide” (Ety/PAT).
• [gm] > [ŋm]: ᴹ√DOƷ > dogme > dongme > ON. doume > N. daw “night-time, gloom” (Ety/DOƷ).

The developments of [gm] > [ŋm] and [gn] > [ŋn] are obscured by the fact that later [ŋ] vocalized before [m] and [n] producing diphthongs, as for example:

• ᴹ√NDAK > ON. ndagno [> ndaŋno > dain] > N. daen “corpse” (Ety/NDAK).

It is not clear when Tolkien limited this phonetic rule to homorganic stops. There is at least one Sindarin example (probably mid-1960s) that indicates non-homorganic stops could still become nasals before nasals:

• ✶ledme-mbasta > lenm(e)mbast(a) > lenmbast > S. lembas “waybread” (PE17/51).

The etymology of lembas was revised several times after this example, so it is not clear whether this was a holdover from the Noldorin etymology of this word, or whether Tolkien still considered [dm] > [nm] a valid sound change.

This sound change was described by David Salo (GS/§4.47) and can be deduced from the examples in The Etymologies. The clearest examples of this change are:

• ᴹ✶stabnē > ON. sthamne > N. thafn “(wooden) post”
• ᴹ✶yatmā > yadme > ON. yanme > N. ianw “bridge” (Ety/ƷEL, YAT)

In the second example, ON. yadme is attested as an element in ON. elyadme “sky-bridge” = “rainbow” (Ety/ƷEL) and its later form ON. *yanme is attested in the plural: yanmī “bridges” (PE22/27). @@@ Later changed to only homorganic stops (VT42/26)

@@@ but see ✶ledme-mbasta > lenm(e)mbast(a) > lenmbast > S. lembas (PE17/51)