I found this answer satisfying:
/j/. Edwin G. Pulleybank wrote in Some Notes on Chinese Historical Phonology:
Vietnamese d and v are also partly derived from earlier *j and *w respectively as we can see from other comparisons with Mương (Thompson 1976). This accounts for the fact that d corresponds regularly in Sino-Vietnamese with LMC j (initial yu 喻 Grade IV). Presumably Vietnamese *j, including Sino-Vietnamese j, had become dʲ in the seventeenth century when the missionary orthography was invented. The phonetic change is comparable to that by which Latin Jacobus has given Diego in Spanish. After this change *j merged with dʲ from softened t. Some modern dialects preserve this pronunciation but in Hanoi the pronunciation is z and in Saigon it is j whatever the origin. It is likely that this is the result of a secondary loss of occlusion since the seventeenth century rather than a continuation of earlier *j.
I thought the comparison to Latin JACOBVS giving rise to Spanish DIEGO was rather brilliant, (everything that follows is an edit/addendum…) although as noted in one of the comments below,
Diego actually comes from
That isn’t to say that it didn’t happen that way. Scottish Gaelic underwent a fortitioning process:
- /v/ → /p/ Scots vervane, werwane ‘vervain’ → bearbhain /pɛɾavɛɲ/
- /ʍ/ → /kʰ/ Scots quhel ‘wheel’ → cuidheall /kʰujəl̪ˠ/
- /w/ → /p/ Middle English wall → balla /pal̪ˠə/
- /f/ → /p/ Latin fundus → bonn /pɔun̪ˠ/ (foundation)
- /θ/ → /t̪ʰ/ Norse þrǣll → tràill /t̪ʰɾaːʎ/ (slave)
- /h/ → /t̪ʰ/ Scots hogsheid ‘hogshead’ → tocasaid /t̪ʰɔʰkəs̪ətʲ/
- /j/ → /kʲ/ English yawl → geòla /kʲɔːl̪ˠə/
The connection between
/j/ can also be found in the Japanese languages. In the book Is Japanese Related to Korean, Tungusic, Mongolic and Turkic?, Martine Irma Robbeets writes:
…the assumption that proto-Japanese had voiced obstruents is stronger in the case of pJ *b and pJ *d due to some modern southern Ryukyu dialects that have b- and d- corresponding to OJ w- and OJ y-. As for the reconstruction of pJ *d, the dialectal evidence rests entirely on a single dialect Yonaguni, which reflects initial OJ y- as d-. The dialectal evidence is rather weak because the Yonaguni d- is restricted to the initial position. Moreover, d- also appears in borrowings from Chinese such as Yo. dasai ‘vegetables’ which reflects a fortition y- > d-. Therefore, I doubt the antiquity of Yonaguni d- and I am reluctant to reconstruct pJ *d- as an ancestor for OJ y-.
Later on, Robbeets writes:
An argument contra the reconstruction of pJ *y is that from the cross-linguistic viewpoint a fortition of a palatal glide to a dental stop is an unnatural sound change, but Miyake mentions a similar development from pre-Middle Vietnamese *y to Middle Vietnamese *d.
It is far more likely, then, that the fortition of
/j/ to a dental happened via some intermediate fricative or affricate. A similar fortitioning process changed the pronunciation of some words borrowed from Chinese from a sibilant to dental stop:
/ð/ was indeed one of the former pronunciations of
<d> as commonly stated, then the sound change might have gone the route:
/j/. The fortition was at least strong enough for the missionaries to assign the phoneme to
<d>. Of course, none of this is for certain and anything without citation is just my pure conjecture.