AQ. [z] and [ð] assimilated to following [r], [l]; [{zð}r|{zð}l] > [rr|ll]

AQ. [z] and [ð] assimilated to following [r], [l]; [{zð}r|{zð}l] > [rr|ll]

In cases where they did not undergo other changes such as metathesis, the primitive combinations dl, dr; sl, sr eventually assimilated to the following liquid and became ll, rr in Quenya. This is not especially surprising, since these combinations first became ðl, ðl (or řl, řr); zl, zr and then both ð, z > r in the Ñoldorin dialect of Quenya, and more generally [rl] became [ll]. However, notes in the Outline of Phonology [OP2] in the 1950s indicate that these sound changes also took place in the Vanyarin dialect:

d became , as between vowels, before w. But dr, dl > rr, ll. In other words, in Quenya, before l, r, w CE d and r became identical ... Note dr, dl > rr, ll also occurred in Vanyarin. The assimilation probably took place at stage řl, řr before V. shift of > z. But sl, sr > zl, zr had same result in V. (and Ñ.) (OP2: PE19/94)
In that case there are three developments of sr: ... (b) after the voicing of intervocalic s > z, sr > zr > rr (such forms were more frequent in N. dialect, since z itself had there become r) ... sl remained as such until the voicing; then zl > ll (OP2: PE19/102).

Since ř, z merged to z instead of ř/r in Vanyarin, the Vanyarin developments indicate the shift to ll, rr took place before the ř/z-mergers of both dialects. Thus, the likeliest phonetic developments were ðl, ðr; zl, zr > ll, rr directly rather than becoming r first.

Conceptual Development: The Early Qenya developments are confused by the fact that ð is part of phonetic inventory of Primitive Elvish in the 1910s and 1920s. Tolkien mentioned no special developments for these combinations in the Qenyaqesta from the 1910s (PE12/24). There are no clear examples of any developments for dl, ðl, dr, ðr in this period, with the possible exception of ᴱ✶nḷdle > ᴱQ. nalle (but here the presence of syllabic confuses the picture).

Tolkien did describe developments for s > z before l, r, however:

s gave z and then r before l, r ... giving ll, ´r (PE12/19)

Tolkien indicates z vanished before r with compensatory lengthening rather than the pair developing into rr, but there are no clear examples. There is one example of sl [> zl] > ll: ᴱ✶kasla > ᴱQ. kalla “helmet” (PE13/140).

In the Outline of Phonetic Development [OP1] from the 1930s, Tolkien did describe special developments for dl, dr:

[d] became [r] as between vowels, before y, w; dr, dl > rr, ll (OP1: PE19/46).

However in this period, it seems that dr, dl > rr, ll is part of the general trend whereby [d] > [r]. There is an example ᴹ✶nyadrō > ᴹQ. nyarro in The Etymologies (Ety/NYAD). Tolkien likewise mentioned these developments in OP1 for sl but not for sr:

While s was still voiceless sr > > ss (similar to developments after stops); but sl appears to have remained as such, and asyllabic. Hence it later became zl > ll (OP1: PE19/49).

There are no examples from the 1930s that demonstrates sr > ss. However, the sound change sr > rr is mentioned in rejected notes from OP1 (PE19/52 note #119), and there is at least one example of it in this period: ᴹ√SRŌ > ᴹQ. orro- “rise (from ground)”, ᴹQ. orro “up in the air, on high” (PE22/127). The best example of zl > ll is the (archaic) perfect of the verb: ᴹQ. hlik- “creep” derived from the root ᴹ√SLIK: izlīk- > †illīk[ie] (PE22/113).

References ✧ PE17/71; PE19/94, 102

Order (04500)

After 02800 voiced stops became spirants except after nasals
After 04400 medial [s] often became [z]

Related

Phonetic Rule Elements

[ðr] > [rr] ✧ PE19/94 (dr > rr)
[ðl] > [ll] ✧ PE19/94 (dl > ll); PE17/71 (-rl- > ll; assimilation)
[zr] > [rr] ✧ PE19/94 (zr > rr); PE19/102 (zr > rr)
[zl] > [ll] ✧ PE19/94 (zl > ll); PE19/102 (zl > ll)

Phonetic Rule Examples

nizle > nille zl > ll nis > Q. nill- ✧ VT47/33
cazrea > carrea zr > rr karrai > Q. carrea ✧ PE22/159
cazrea > carrea zr > rr cas-raya > Q. carrea ✧ VT42/12
mizroanwe > mirroanwe zr > rr mi-srawanwe > Q. Mirröanwi ✧ MR/350
mizroanwe > mirroanwe zr > rr mi-srawanwe > Q. Mirröanwi ✧ MR/350

ᴹAQ. [z] and [ð] assimilated to following [r], [l]; [{zð}r|{zð}l] > [rr|ll]

References ✧ PE19/46, 49

Order (04600)

After 02900 voiced stops became spirants except after nasals and liquids
After 04500 medial [s] often became [z]

Phonetic Rule Elements

[ðr] > [rr]
[ðl] > [ll]
[zr] > [rr]
[zl] > [ll] ✧ PE19/51 (zl > ll)

Phonetic Rule Examples

izlīk- > illīk- zl > ll ᴹ√SLIK > izlīk- > ᴹQ. †illīk[ie] ✧ PE22/113
sizlikk- > sillikk- zl > ll ᴹ√SLIK > sizlikk- > ᴹQ. †sillikk- ✧ PE22/113
ozro- > orro- zr > rr ᴹ√SRŌ > ᴹQ. orro ✧ PE22/127
ozro > orro zr > rr ᴹ√SRŌ > ᴹQ. orro ✧ PE22/127
njaðrō > njarrō ðr > rr ᴹ✶nyadrō > ᴹQ. nyarro ✧ Ety/NYAD

ᴱQ. [rl] became [ll]; [rl] > [ll]

Reference ✧ PE12/19

Order ()

After [z] became [r]

Phonetic Rule Elements

[rl] > [ll]

Phonetic Rule Examples

karla > kalla rl > ll ᴱ✶kasla > ᴱQ. kalla ✧ PE13/140