Q. [rs] became [ss]; [rs] > [ss]
In Tarquesta [TQ] pronunciation there was a tendency to unvoice r before s and simplify the pair to ss. Supposedly this effected r of all origins, and also s < þ. Tolkien mentioned this phonetic development in both the Outline of Phonetic Development [OP1] from the 1930s and the Outline of Phonology [OP2] from the 1950s:
ls, rs were preserved in PQ and so in correct spelling. But the Lindarin pronunciation of rs (not rs < rth!) as ss was frequent in TQ pronunciation (OP1: PE19/47).
rs remained in PQ, but the phonetic tendency in the spoken language was evidently to complete unvoicing and reduction of the group > ss. The tendency was still operative in TQ after the TQ change of th, þ > s. Both were sounded ss in colloquial TQ, but since the traditional spelling was retained rs was still used by the “learned” and regarded as the correct usage. ls normally was unchanged (OP2: PE19/99).
In 1930s Tolkien said this was a Lindarin-only change and did not affect rth, though elsewhere in OP1 he said “there was a strong tendency for rþ > rs to become in TQ ss (PE19/45)”. In OP2 rs > ss was a TQ change. Since the combination rþ > rs > ss in the 1950s, perhaps it was a Ñoldorin-only change, but more likely this sound change remained active for some time and simply applied to s of all origins. In notes from the mid-1960s, Tolkien also said this sound affected r of any origin:
r of any origin tended to become partly or wholly unvoiced before t, p, k, s, and rs was confused with ss (PE17/71).
Despite the above, the combination rs is reasonably common in Quenya, but where it appears it is almost always the result of rþ < rth:
The few case where we see rs > ss, they are all combinations of original primitive r and s:
It is conceivable that these surviving rs represent “preserved spelling”, but it is also possible that, despite Tolkien’s statements in OP1 and OP2, this was a late PQ or early TQ sound change before þ > s, and only affect original primitive combinations of rs and not r or s of other origins.
Also note that despite Tolkien’s statement that ls survived, that combination is rare in Quenya and there is at least one example where it seems ls > ll:
Conceptual Development: The combination rs does not appear in Early Quenya, but in Qenyaqetsa from the 1910s Tolkien said “ls, rs > rs, ls; rarer combinations” (PE12/19), which makes it seem like rs survives. Lack of examples makes it hard to determine when he decided rs > ss.
References ✧ PE17/71; PE19/99
Phonetic Rule Elements
||✧ PE17/71 (rs > ss); PE19/99 (rs > ss)|
Phonetic Rule Examples
|nersa > nessa||rs > ss||✶neresā > Q. Nessa||✧ WJ/416|