In Sindarin and Noldorin, the combinations [mb] and [nd] became [mm] and [nn] medially and (usually) finally, whereas [ŋg] remained medially but became [ŋ] finally. Tolkien described these sound changes in some detail in The Lord of the Rings appendices:

In Sindarin the combinations ng, nd, mb, which were specially favoured in the Eldarin languages at an earlier stage, suffered various changes. mb became m in all cases, but still counted as a long consonant for purposes of stress (see below), and is thus written mm in cases where otherwise the stress might be in doubt. ng remained unchanged except initially and finally where it became the simple nasal (as in English sing). nd became nn usually, as Ennor “Middle-earth”, Q. Endóre; but remained nd at the end of fully accented monosyllables such as thond “root” (cf. Morthond “Blackroot”), and also before r, as Andros “long-foam”. This nd is also seen in some ancient names derived from an older period, such as Nargothrond, Gondolin, Beleriand. In the Third Age final nd in long words had become n from nn, as in Ithilien, Rohan, Anórien (LotR/1115).

As this note indicates, the reduction of [mb] was universal, but the change of [nd] into [nn] sometimes did not occur, notably “at the end of fully accented monosyllables”. Tolkien mentioned this retention of nd in stressed monosyllables elsewhere: “In Sindarin this gave nand which as other words ending in nd remained in stressed monosyllables but > nann > nan in compounds” (NM/351).

The simplification of mb, nd is mentioned in a variety of other places in Tolkien’s writings (PE17/104, PE19/92), but the only other lengthy discussion appears in his notes on the Noldorin use of the Feanorian Alphabet from the 1930s (PE22/35-36):

The groups mb, nd, ŋg written t 5 g have now become [m]; [nn] medially and [n] finally; [ŋg] medially and [ŋ] finally. The spelling largely remained unchanged. The process was as follows:

Final [mb] became [m] in earliest Gondolic, probably before Y.S. 300. Hence t and y interchange finally, y transcribed m predominating: thus Gond. lham “tongue”, Nargothrondic and Feanorian dialects lhamb until later.

Final [ȵg] similarly, but perhaps later, also became [ŋ] before the end of the Gondolic period, but since the alphabet possessed no sign for isolated [ŋ], the spelling with g, transcribed ng, remained unchanged.

Medial and final [nd] remained unchanged until late Exilic. Hence such spellings in the histories as Grond, Angband, Beleriand, Gondolin, &c.

In later Gondolic (after Y.S. 300) medial mb > mm. The letter t thus became used = [mm] of any origin (PQ pm, bm, sm, mm, mb) and is usually transcribed mm. Thus w]F7 [bār] “home” transcribed bár, pl. `B=tl`B7 [imméı̯r later iméı̯r] transcribed i·mmeir. Before or after consonants in compounds &c., [mm] > [m], usually recognized in spelling: so archaic amb “up” + rhūn “rising” > [ambrūn > amrūn]: written ]y7.F6, transcribed amrún.

In post-Exilic, Toleressean, [mm] generally > [m] but this was not recognized usually in spelling.

In late Exilic (after Y.S. 307) [nd] > [nn]. The letter 5 thus became used of [nn] of any origin (PQ tn, dn, sn, nn, nd); and is usually transcribed nn, except in ancient historical Gondolic names: as Gondolin. This [nn] became [n] finally and before consonants, and this is recognized in spelling, except that 5 = nn is retained in stressed monosyllables as a sign of the short quality of the preceding vowel. Thus sh5 = gŏnn, Gondolic gond “rock”.

Note ndr > nr not đr (as nr, nnr, see Consonants): So Belerian(d)ren “Beleriandic” but Gondoliðren “Gondolic”. In post-Exilic [nn] > [n] before a main stress and so also in initial mutation (of d); but this was not recognized usually in spelling: the mutation of 2 [d] is always written 5 [nn or nd archaically]. Thus 2]F7 dór “land”, pl. `=5®R7 i·nnýr [inȳr] “the lands” [per editors note #116, dór should be 2hR7].

In late Exilic [ŋg] suffered no further change. g thus represents [ŋg] except finally, where it is [ŋ]. [ŋg] has since remained before r, l, but became [ŋ] before other consonants, as Angband = [aŋban]. Before a main stress [ŋg] is sometimes in Toleressean reduced to [ŋ], but this is not normal.

Note that mb, nd, remaining, because analysed or separated syllabically, or redeveloped — as in compounds of various dates — were, after the development of 5 t as signs for [nn, mm], written 2P wP. Similarly [ŋg] in which [ŋ, n] belongs to one word, or component, is usually written sP.

The sound changes described here are mostly compatible with the description in The Lord of the Rings, written almost three decades later. Combining the information in these two notes, the order of sound changes seems to be roughly as follows:

1. All combinations ([mb], [nd], [ŋg]) generally shortened to a simple nasal before another consonant.
2. [-mb] > [-m] finally, and shortly afterwards [-ŋg] > [-ŋ].
3. Medial [mb] became [mm].
4. Medial and final [nd] became [nn].
5. Final [nn] became [n] (in polysyllables).
6. Medial [mm] became [m], preserved sometimes in spelling to indicate stress patterns.

It seems Noldorin and Sindarin had somewhat different developments for final nd/nn. According to the notes in the Feanorian Alphabet, in Noldorin nn always shortened finally, but was retained in spelling for monosyllables to reinforce the fact that the preceding vowel was short; this spelling convention makes it difficult to detemine how rigorously Tolkien adhered to this rule. In Sindarin it seems that nn consistently shortened only in polysyllables, and sometimes nd was retained in monosyllables when “stressed” (see below for further discussion).

In both conceptual periods, there were some special conditions where the longer clusters [nd] and [ŋg] were preserved, notably before liquids [r], [l]. Despite Tolkien’s statement above from the Feanorian Alphabet document, the combination ndr appears quite frequently in Noldorin as it does in Sindarin. The combination ndl does not appear in either Sindarin or Noldorin. This may be a coincidence, but Tolkien explicitly mentions the combination ndr and not ndl in The Lord of the Rings appendices (LotR/1115), so it seems likely the latter combination would reduce to nl. Developments for [ŋg] are somewhat harder to unravel, since Tolkien used the spelling ng for both [ŋg] and [ŋ], both in Latin characters and in Tengwar. However, both ngl and ngr appear in Noldorin and Sindarin, consistent with Tolkien’s statements above that this combination was preserved before liquids. The combination mbr also appears in a few cases as well (such as S. Fimbrethil and S./N. Nimbrethil), hinting that even mb could be preserved before r, though here mr was much more common.

These combinations appear regularly in compounds, so it seems that (a) larger clusters could prevent this change (Elmbereth > Elbereth) and (b) this sound change was no longer active in “modern” Sindarin (hat tip to Elaran for pointing out their regular survival in many compounds):

• S. andaith “long-mark” (PE17/123), N. andeith (Ety/TEK).
• S. randir “wanderer” (PE17/60, VT42/13), N. rhandir (Ety/RAN).
• S. and N. Calembel “Greenham” (LotR/790, RC/537, WR/437), a name from the Third Age.
• S. and N. lembas “waybread” (LotR/369, EtyAC/LEN).

In Sindarin, where final -nd was preserved sometimes in monosyllables, it could also reappear in later compounds by analogy with isolated elements, notably in the Third Age name S. Morthond “Black Root” which contained thond “root” (LotR/1115). This was sufficiently common that Sindarin speakers had little difficulty with archaic names from the First Age that ended in -nd. This was not the case for -mb; that combination had disappeared finally long enough ago that it doesn’t show up finally even in ancient names.

Preservation of final -nd in Sindarin monosyllables: This leaves only the question of when final nd was preserved “at the end of fully accented monosyllables” in Sindarin, a fact Tolkien mentioned again in notes from the late 1960s:

The element -nan “valley” was derived from C.E. base NAD “hollow” ... In Sindarin this gave nand which as other words ending in nd remained in stressed monosyllables but > nann > nan in compounds (NM/351).

In Noldorin of the 1930s, monosyllables usually show final [-nn], with a handful of exceptions: N. grond “club”, N. gwend “bond, friendship”, N. ind “bond, friendship”, N. lhand “open space, level”, N. lhend “tuneful, sweet” (from various places in The Etymologies). There are many Sindarin monosyllables that show only final [-nd], however, including a number of cases where Noldorin shows [-nn]:

• S. band “prison” (MR/470) vs. N. bann (Ety/MBAD).
• S. gond “rock” (WJ/201) vs. N. gonn (Ety/GOND).
• S. gwen(d) “maiden” (PE17/191) vs. N. gwenn (Ety/WEN).
• S. lond “haven” (UT/264) vs. N. lhonn (Ety/LOD).
• S. rond “(vaulted) roof, cave” (RC/421; WJ/414) vs. N. rhonn (Ety/ROD).

However, often Sindarin monosyllables have variants ending in [-nn]. For example, S. and “long” had a variant form ann (PE17/12, 40, 121), and in one place Tolkien said:

S. †ann- [“long”] only preserved in certain compounds owing to competition with ann (< annā) “gift”, and ann(on) “gate”, of different origin (PE17/90).

This only makes sense if the normal form of the monosyllable and “long” was in fact ann. There are many cases where Sindarin show both -nd, -nn variants, sometimes with the -nd variant explicitly marked as archaic (†):

• S. brand and brann “towering, tall and massive” (PE17/022-23, 61).
• S. grond and gronn “very weighty and ponderous” (PE17/99).
• S. lind and linn “chant, song” (PE17/17).
• S. †pend “steep incline, slope” (PE17/24) vs. ordinary penn (PE17/173; RC/525).
• In one place, S. †gond and gonn “stone” (PE17/29), elsewhere usually given as S. gond.
• S. inn “purpose or intention” (PE22/165) vs. S. ind as an element in S. Saelind “Wise-heart” (MR/305).

Where Noldorin -nd and -nn variants appear together in The Etymologies, it is clear the -nd form is archaic and the -nn modern, so this is probably true as well for the Sindarin examples above. Thus, it seems Tolkien was not entirely committed to the idea that -nd was preserved in Sindarin monosyllables, and sorting out exactly actually when and why -nd was preserved is very difficult.

Conceptual Developments: As noted by Roman Rausch in his Historical Phonology of Goldogrin (HPG/§2.3), these sound changes were already mostly established in the Gnomish of the 1910s, with some indications that generally [nd] > [nn] medially:

• ᴱ✶ṇ̄dai > G. annai “far away” (GL/19).
• ᴱ✶sind(an)ai > G. sinnai “today” (GL/68) appearing beside (deleted) sindan.
• G. indos and innos “house, hall” (GL/19).
• G. nendil and nennil “waterfay” (GL/60).

The same is not true of [mb]; all the obvious examples with the reduction of [mb] > [m] appear before another consonant:

• ᴱ✶ambalatā > G. amlad “surface” (PE13/109).
• G. am(b)ros(t) “dawn” = am¹ + rost (GL/19).

Furthermore, there are a fair number of examples of medial mb that do not appear to be the result of compounds:

• G. gwembel “weevil” beside gweml(i) (GL/45).
• G. tambos “cauldron” vs. G. tam “copper” (GL/69).
• G. tumbol “valley-like, hollow, excavated” vs. G. tûm “valley” (GL/71).
• G. umbart “fate” beside umrod (GL/56, 75).

At the end of words, though, it seems that [-mb] > [-m] and [-nd] > [-nn] (and to [-n] in polysyllables) fairly regularly:

• [deleted] ᴱ✶kamba > G. cam “bent, bowed” (GL/25).
• G. tûm “valley” vs. ᴱQ. tumbo (GL/71; QL/95; LT1A/Tombo).
• G. bron(n) “steadfast, firm” vs. ᴱQ. voronda (GL/24; QL/102; LT1A/Bronweg).
• ᴱ✶anda > G. [a]nn “door, opening” (PE13/110).
• ᴱ✶glon-χend > G. glonthen “dandelion” (GL/40).

There are, however, a fair number of Gnomish monosyllables ending in -nd, so it seems Gnomish had the same -nd/-nn variation as Sindarin for monosyllables. There are also a fair number of Gnomish clusters with ndr, so it seems that other than medial survival of mb, the Gnomish developments of the 1910s were quite similar to Sindarin. Developments in Early Noldorin are more similar still, since medial [mb] started to become [m] in the 1920s:

• ᴱ✶a-mbos-t > ambost > amboth > ᴱN. amoth “shoulder” (PE13/159).
• ᴱ✶a-mbṝt- > ᴱN. amarth “fate” (PE13/159).

Neo-Sindarin: It is common among Neo-Sindarin writers to assume that in Sindarin, final -nd should be the normal form based on Tolkien’s statements in The Lord of the Rings appendices, and some authors go so far as to revised Noldorin forms with primitive nd so that they end in -nd rather than -nn in (Neo) Sindarin. For example, see ᴺS. lhind “fine, slender” as an updated form of N. thlinn from Hiswelókë’s Sindarin Dictionary (HSD) and Gateway to Sindarin (GS/271). I personally find this approach to be useful, since it allows us to more easily distinguish words like S. and “long” from S. ann “gift”. This lexicon consistently presents monosyllables with -nd as the “default” in Sindarin, though it does show the -nn variants in cases where Tolkien provided them.

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HSG/§2.3

HSG/§2.3