S. medial [s] became [θ] before [l], [r]; [Vs{lr}] > [Vθ{lr}]

S. medial [s] became [θ] before [l], [r]; [Vs{lr}] > [Vθ{lr}]

Medial s became th (þ or [θ]) before liquids l and r in both Sindarin and Noldorin, as noted by David Salo (GS/§4.144). There are examples in both Sindarin and Noldorin:

One interesting case is N. Cílu < ᴹ✶χīsislōmē (PE21/32). Here it seems the intervocalic [s] became [h] so that after short vowels vanished before morpheme boundaries the result was the cluster [hsl]. This cluster either inhibited or masked the sound change [sl] > [θl], so that the result was [kīlu] after [h] vanished after vowels.

Conceptual Developments: This sound change dates back to the Gnomish of the 1910s, as pointed out by Roman Rausch in his Historical Phonology of Goldogrin (HPG/§2.5). A couple of representative examples:

Order (04900)

After 03900 nasals vanished before spirantal clusters ᴹ✶la(n)sro-ndo > N. lhathron Ety/LAS²
After 04800 middle consonants frequently vanished in clusters ᴹ√LOD > othlond > N. othlon
ᴹ√LUS > N. lothren
Ety/LOD
EtyAC/LUS

Related

Phonetic Rule Elements

[Vsl] > [Vθl]
[Vsr] > [Vθr]

Phonetic Rule Examples

kasrae > kaθrae Vsr > Vθr cas-raya > S. cathrae ✧ VT42/12

N. medial [s] became [θ] before [l], [r]; [Vs{lr}] > [Vθ{lr}]

GS/§4.144, note: Cílu @@@

Order (04600)

After 03700 nasals vanished before spirantal clusters ᴹ✶la(n)sro-ndo > N. lhathron Ety/LAS²
After 04500 middle consonants frequently vanished in clusters ᴹ√LOD > othlond > N. othlon
ᴹ√LUS > N. lothren
Ety/LOD
EtyAC/LUS
Before 05300 [mb], [nd] became [mm], [nn] ᴹ√LOD > othlond > N. othlon Ety/LOD

Related

Phonetic Rule Elements

[Vsl] > [Vθl]
[Vsr] > [Vθr]

Phonetic Rule Examples

oslond > oθlond Vsl > Vθl ᴹ√LOD > othlond > N. othlon ✧ Ety/LOD
l̥asra- > l̥aθra- Vsr > Vθr ᴹ√LAS² > N. lhathro ✧ Ety/LAS²
l̥asrond > l̥aθrond Vsr > Vθr ᴹ✶la(n)sro-ndo > N. lhathron ✧ Ety/LAS²
losren > loθren Vsr > Vθr ᴹ√LUS > N. lothren ✧ EtyAC/LUS
osrond > oθrond Vsr > Vθr ᴹ√OS > N. othrond ✧ Ety/ROD

G. [s] became [θ] before [l], [r], [w]; [s{rlw}] > [θ{rlw}]

HSG/§2.4

Order (05800)

Before 05900 initial [θ] sometimes became [f] before [l], [r], [w]

Related

Phonetic Rule Elements

[sl] > [θl]
[sr] > [θr]
[sw] > [θw]
[tl] > [θl]

Phonetic Rule Examples

mbosli > mboθli sl > θl ᴱ✶mᵇāsḷ > G. bothli ✧ GL/23
sloss > θloss sl > θl ᴱ√thol- > G. thloss ✧ GL/73
etlin > eθlin tl > θl ᴱ✶etḷ- > G. ethl(in) ✧ GL/33