In both Sindarin and Noldorin, combinations of homorganic nasals and voiceless spirants became long nasals: mph, nth, ŋch > mm, nn, ng (the long nasal ŋŋ became ŋg). The long nasals were initially voiceless, but were later voiced. Tolkien described this sound change in several places, often lumped together with similar changes for lth:

Medially however nth (nþ), nch (ñx), mf (mp with bilabial f) and lth (lþ) became long voiceless n, ñ, m, l though the old spelling was mostly retained (beside nh, ñh, mh, lh), and among those to whom Sindarin became a language of lore, as were the men of Gondor who were or claimed to be of the Númenórean race, the spirant was reintroduced from the spelling. In true Sindarin of the Elves or Elf-friends of the early ages the final form was often reintroduced medially (The Rivers and Beacon-hills of Gondor, 1967-69, VT42/27).

Medially lþ, mph, nþ, ŋχ (though for long without change of spelling) became long voiceless l, m, n, ŋ, English transcription lh, mh, nh, ngh. In late Beleriandic Sindarin these became voiced ll, mm (m), nn, ng (= ŋg) unless followed still by a stressed syllable (phonetic notes from 1962, PE17/131).

In addition for voiceless n 6J ({nhui >>} anhau) was used, and [for voiceless] m yJ ({mhui >>} amhui) [was used] (The Feanorian Alphabet late 1930s, PE22/30 note #98).

For nasal combinations there are plenty of examples of this sound change in both Noldorin and Sindarin. Some representative examples:

• ✶mantinā > manthen > S. mannen, passive participle of S. mad- “to eat” (PE17/131).
• ✶dankĭnā > daŋχen > S. danghen > S. dangen “slain” (PE17/133).
• ON. gampasse > N. gammas “s-sign” (EtyAC/GAP).
• ON. thintha > N. thinna “fade” (Ety/THIN).

Conceptual Developments: The medial combinations mf, nth, nch were fairly common in the Gnomish of the 1910s, so it seems this sound change was not part of Tolkien’s earliest ideas for the language. As noted by Roman Rausch in his Historical Phonologies of Ilkorin, Telerin and Noldorin around 1923 (HPITN/§4.1.5), there are examples of medial combinations nc, nt, mp changing to (long) nasals (probably via mph, nth, ŋch) starting with the Early Noldorin of the 1920s:

• ᴱN. engin plural of ᴱN. anc “spear” (PE13/137, 159).
• ᴱN. ennir plural of ᴱN. ant “face” (PE13/137, 160).
• ✶rankind- > ranχin > reı̯ngi(n), a verb form of ᴱN. rhang- “to slay” (PE13/152).

But there are several examples of medial nth in this period as well: ᴱN. cranthir “perfect” (PE13/161), ᴱN. huntha- “to burnish, polish” (PE13/148), ᴱN. panthig “coin” (PE13/124). It seems that Tolkien had not yet firmly decided on this sound change, or perhaps there were some less-than-obvious special conditions that inhibited this phonetic development.

Abandonment of the Voiceless Nasal Stage: There is some evidence that Tolkien abandoned the sound changes that produced long (voiceless) nasals from nasals + voiceless spirants at various points later in his life. In Quenya Notes (QN) from 1957 Tolkien wrote:

√STEN-. Since nt &c. no longer are held to yield nn &c., the word for “short” should be ✶stenna, √STEN- “cut short, limit, confine, cramp”. Q thenna/senna. S thenn, esten (PE17/185).

This note hints that nt > nth > nh > nn is no longer a feature of Sindarin, requiring a change to the form of the Sindarin word for “short”: thent >> thenn [< ✶stenna]. This 1957 idea was apparently transient, since nt > nth > nh > nn appeared in notes from 1962 (PE17/131) and the late 1960s (VT42/27) as described above.

However there are a few examples of preserved nth and nch in past-tenses from Tolkien’s writings in 1969 (echanthel VT47/38 and tancher/adancher PE23/138 note #12), indicating he may have once again abandoned this sound change. The relevant analysis is quite complex, since it is tied to past-tense forms and how nasal mutation applies to voiceless stops. I discuss this topic at length within my own analysis of the sound changes in Tolkien’s 1969 notes on the Common Eldarin Article (CEA): see my essay Sindarin Articles and Mutations from Parma Eldalamberon #23, section 4.0.

GS/§4.112 GS/§4.118 GS/§4.201 @@@ but not -lθ- < -lt-: heltha-