In Sindarin and Noldorin, voiceless stops became voiced stops before nasals. There is a clear description of this sound change in notes associated with The Rivers and Beacon-hills of Gondor from the end of the 1960s:

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) (VT42/26).

It is hard to find obvious Sindarin examples of this sound change, but there are a fair number of examples in Noldorin:

• ✶okma [> *ogma > *oim] > S. oew “crime” (PE17/170).
• ᴹ✶rakmē > ON. ragme > N. rhaew “fathom” (Ety/RAK).
• ᴹ√NDAK > ON. ndagno > N. daen “corpse” (Ety/NDAK).
• ᴹ✶lepenar [> *lebnar] > N. lhevnar “week” (EtyAC/LEP).
• ᴹ✶yatmā > ON. yadme > N. ianw “bridge” (Ety/ƷEL, YAT).
• ᴹ✶patnā [> *padna] > N. pann “wide” (Ety/PAT).

As shown by these examples, the sound change must have occurred in Old Noldorin, and David Salo attributes the change to Old Sindarin in his Gateway to Sindarin (GS/§4.46). There are a few similar sound changes in Welsh, notably [tn] > [dn], but there the changes are less comprehensive (WGHC/§104i).

Conceptual Development: There are not enough examples in Gnomish or Early Noldorin to determine what these developments were at that conceptual stage of the Early Period.

GS/§4.46 @@@