One of the more characteristic phonetic developments of Quenya is that the palatalized velars ñy, ky, gy changed into dental ny, ty, dy. There was a similar change in Telerin, but since Sindarin shows a different development ([j] was lost after initial velars) this must have after the Ancient Telerin stage. Thus, the dentalization of palatalized velars must have been a parallel rather than common development in Quenya and Telerin, though conceivably the two languages could have influenced each other.

The Quenya phonetic developments are mentioned throughout both the Outline of Phonetic Development (OP1) from the 1940s and the Outline of Phonology (OP2) from the 1950s (OP1: PE19/34-35, PE19/40, PE19/43, PE19/46; OP2: PE19/74-75, 77, 83, 86, 92-93, 96-97). The most complete descriptions in the 1930s are:

Initial groups ... The combinations with y: ky, khy, gy, ñy. ñy became ny (at the time ky > ty) (ny as in English “new”). khy became χy with [χ] fronted. The ultimate result was a strong voiceless y [ɧ], here transcribed hy — similar to (but more strongly spirantal than) English “huge”. ... gy became spirantal y, but this fell in, in PQ, with original y. In ky the k was fronted. The result was transcribed ty. The sound is closely similar to “t” in English “tune” (OP1: PE19/33-34).

[Medial] k, g were fronted to t, d before y. Owing to the opening in Q. of all voiced stops (except after nasals and l, r) gy > dy only occurred in ndy, ldy, rdy (< ñgy, lgy, rgy). On the pronunciation of ty, dy see below. Medial gy, however, became ʒy > yy > iy. Note khy did not become thy, but produced [χy] with fronted [χ] a strongly spirantal (long) voiceless [y], [ɧ] transcribed hy (OP1: PE19/40).

As described above, dentalization occurred medially and finally, but only after the spirantalization of both aspirates and voiced stops. The net result is that only ky, ñy consistently became ty, ny (as well as ñky > nty); khy never dentalized (instead becoming hy) and gy only dentalized in tri-consonantal clusters where it failed to spirantalize: ñgy, lgy, rgy > ndy, ldy, rdy.

The medial phonetic developments in the 1950s were exactly the same (PE19/83), but the initial developments were different because spirantalization happened at a later stage for initial voiced stops:

The dentalization of k-series before (initial) y was early: it is also shared by Telerin. Thus gy > dy or đy (if the development was after the spirantalization of the voiced stops). The result in any case was ly ... Medial gy became ı̯y, which coalesced with preceding vowels or formed diphthongs: thus CE magyā > maiya, maia. This was because opening of the medial stops was much earlier than in case of the initials; so that gy already to ʒy > ı̯y before dentalization (OP2: PE19/75).

The net result is that initial gy- did dentalize, and became gy- > dy- (> ?ðy-) > ly-. This splitting of spirantalization into two phases first appeared in revisions to OP2 (PE19/75 note #37) and almost certainly coincides with the introduction of the new initial cluster ly into Quenya (PE19/80 note #54). See the discussion on how [dj] became [lj] for more details.

Conceptual Development: In the 1910s and 1920s, Primitive Elvish had a pure palatal series (PE12/15). In Qenya, the palatals [ɲ], [c], [ɟ] became more dental in nature, becoming ny, ty, dy (PE12/16), which is how they were represented in roots in the Qenya Lexicon. In most case, though, initial dy- became Early Qenya y- (QL/105-6). This is very similar to Tolkien’s later conception of the phonetic development of palatalized velars.

Neo-Quenya: As discussed under the entry on how [dj] became [lj], I recommend ignoring Tolkien’s split in the 1950s of spirantalization into two phases and simply assume it occurred before the dentalization of palatalized velars in all cases. As a result, gy- > y- (the rule in the 1930s).