S. [nr] became [ðr]; [nr] > [ðr]

S. [nr] became [ðr]; [nr] > [ðr]

In both Sindarin and Noldorin, where n and r came into contact in compounds after the loss of vowels at morpheme boundaries, the n became dh. Tolkien actually mentioned this phonetic rule in The Lord of the Rings appendices:

DH represents the voiced (soft) th of English these clothes. It is usually related to d, as in S. galadh “tree” compared with Q. alda; but is sometimes derived from n+r, as in Caradhras “Redhorn” from caran-rass (LotR/1113).

Tolkien mentioned this change in several other places, including:

Elen, pl. Elin, with class-plural Eledhrim, Elf, Elves. dhr is < n-r in secondary contact (Quendi and Eldar, circa 1960, WJ/363).
Note ndr > nr not đr (as nr, nnr, see Consonants): So Belerian(d)ren “Beleriandic” but Gondoliðren “Gondolic” (Feanorian Alphabet, 1930s, PE22/36).

This last quote indicates this change did not occur where there was another consonant between the n and r (later lost). There are a number of Sindarin and Noldorin words with nr, indicating that [nr] > [ðr] was no longer an active phonetic rule:

As such, this rule only applied in older names and compounds:

Conceptual Developments: There are a couple examples of this sound change in the Gnomish of the 1910s and Early Noldorin of the 1920s:

It seems Tolkien adopted this sound change very early and retained it thereafter.

References ✧ LotR/1113; PE17/36; WJ/363

Order (03700)

Before 05400 middle consonants frequently vanished in clusters findarātō > find-raud > findrod > S. finrod VT41/9

Phonetic Rule Elements

[nr] > [ðr] ✧ LotR/1113 (n+r > [dhr]); PE17/36 (nr > ðr); WJ/363 (n-r > dhr)

Phonetic Rule Examples

karanrass > karaðrass nr > ðr S. caran-rass > S. Caradhras ✧ LotR/1113
karanrass > karaðrass nr > ðr S. caran-rass > S. Caradhras ✧ PE17/36
karanrass > karaðrass nr > ðr S. caran-rass > S. Caradhras ✧ SA/caran
karanrass > karaðrass nr > ðr S. caran-rass > S. Caradhras ✧ TI/174

N. [nr] became [ðr]; [nr] > [ðr]

GS/§4.128 @@@

Reference ✧ PE22/36 ✧ for example: Belerian(d)ren “Beleriandic”, Gondoliðren “Gondolic”

Order (03300)

Before 04800 middle consonants frequently vanished in clusters ᴹ✶Phinde-rauto > N. Finrod Ety/PHIN

Phonetic Rule Elements

[nr] > [ðr]

Phonetic Rule Examples

īnrind > īðrind nr > ðr ᴹ√YEN > iðrind > N. iðrin ✧ Ety/RIN
īnrind > īðrind nr > ðr ᴹ✶ı̯en-rinde > N. îðrin ✧ Ety/YEN
maenros > maeðros nr > ðr ON. Maenros > N. Maedhros ✧ Ety/MAƷ|MAG
onril > oðril nr > ðr ᴹ✶onrō > N. odhril ✧ Ety/ONO
onrond > oðrond nr > ðr ᴹ✶onrō > N. odhron ✧ Ety/ONO