S. long voiceless spirants shortened; [θθ|xx] > [θ|x]
In both Sindarin and Noldorin, long voiceless spirants shortened, as noted by David Salo (GS/§4.184). Tolkien alluded to this phonetic development in a discussion of the transcriptions used by Ælfwine in his document on the Feanorian Alphabet from the 1930s:
S. [h] vanished after vowels; [Vh] > [Vø]
In both Sindarin and Noldorin, any non-initial [h] vanished after vowels, as noted by David Salo (GS/§4.100, GS/§4.107, GS/§4.116). A similar sound change occurred in Welsh (WGCH/§94ii). This sound change primarily applied to the [h] that developed from intervocalic [s] which therefore ultimately vanished, as described by Tolkien in a note in The Etymologies from the 1930s:
*Minaþurindor *entankatánier sa i *rando Silindo láne illume silúmea nómeryasse mi i *Anarya *Panasta.
Nóna ve helka rando amna *kanallume háya pella silúmea nómerya, Silindo rongo *menune alta lendaryanna ya xéyane tere koranari ototuxa húme tenna anyane i nóme imbe Karnil ar *Kormanil. Si enwa entankataina ló i kendare *pityarandoron kombion atta i *pepellar Anar epe ar nó Silindo imya rindesse. Tana pityarandoron ére áne i *sinwasse pá i rando ranya.
S. [ou] became [au]; [ou] > [au]
In both Sindarin and Noldorin, at a late stage of the language, the diphthong [ou] became [au], as noted by David Salo (GS/§4.110). This development is more straightforward in Noldorin, where primitive [ou] also ultimately developed into [au]:
S. final [w], [v] vanished after [u]; [-u{vw}] > [-u]
In Sindarin and Noldorin, any final [v] or [w] after the vowel [u] vanished, as noted by David Salo (GS/§4.173, GS/§4.176, GS/§4.177). The likely mechanism was [v] > [w], and then [w] became [u] which blended with the preceding vowel. This sound change also occurred after the diphthong au, as noted by Tolkien himself:
S. final [v] became [w] after [i], [ae], [oe]; [-{ae|oe|i}v] > [-{ae|oe|i}w]
S. [mm] shortened; [mm] > [m]
S. non-initial [m] usually became [v]; [Vm|{lr}m|m{mbp}] > [Vv|{lr}v|m{mbp}]
In Sindarin and Noldorin, an [m] appearing after a vowel or liquid almost always developed into a labial spirant [v], the only major exceptions being the combinations mm, mb, mp and mph. This was a factor in the soft-mutation of Sindarin, where words beginning with m have mutated forms beginning with v:
S. final and initial [ŋg] became [ŋ]; [ŋg-|-ŋg] > [ŋ-|-ŋ]
In Sindarin and Noldorin, the combination [ŋg] became [ŋ] initially and finally, a sound change Tolkien mentioned in The Lord of the Rings appendices:
S. final [ll], [nn], [ss] shortened in polysyllables; [-SS{ll|nn|ss}] > [-SS{l|n|s}]
In both Sindarin and Noldorin, the long consonants [ll], [ss] and [nn] generally shortened at the end of polysyllabic words. Helge Fauskanger examined these phonetic developments in detail in a pair of articles, To SS or not to SS and The Question of nd or n(n) (on the Ardalambion website). Tolkien himself mentioned these developments in The Lord of the Rings appendices:
