When did the sounds of 'w' and 'v' change in High German?

Question

As far as I know, the sound 'w' is always pronounced as 'v', and 'v' as 'f' in German words, relative to their cognate English words. So my questions, why did these sounds shift, and when? As far as I know, neither shift is die to the High German Consonant Shift, though I could be mistaken.

Answer 1

The change of /w/ to /v/ is not considered to be part of the High German Consonant Shift. It looks like it probably occurred later than that; it seems safe to date it after 500 AD and potentially as late as after the 16th century. Actually, we may not be able to give a specific date of completion as the sound change may still not have spread to all present-day dialects of German:

The glide [w] can also surface in the onset in RG [RG = Ramsau German] as an alternate pronunciation of /v/ and intervocalic /b/. For example, the word Wasser ‘water’ can be pronounced as either [wo.sɐ] or [vo.sɐ]. Variation of /v/ occurs within speakers, so one speaker might produce a word like Wasser with [v] one time and then with [w] the next. This pronunciation of RG [w] reflects an earlier stage of German (i.e. MHG). /v/ is consistently pronounced as [w] in other varieties of Bavarian, such as Imst German described by Schatz (1897:7-8); interested readers are also referred to discussion of Westphalian German in Hall (2014). Intervocalic /b/ also surfaces as [w] in words such as selber ‘self’: [sɛ.wɐ] ~ [sɛ.bɐ]. For more discussion of this, see Zehetner (1985:85) and Merkle (2005:28 §24).

(Erin Noelliste, 2017, "The Phonology of Sonorants in Bavarian German", page 92, footnote 37, edited to add note "[RG = Ramsau German]")

Old High German is reconstructed as having the approximant /w/ by Wikipedia (which I think helps explain the spelling "W"). This may also have been the value in Middle High German:

The most authoritative scholarly work on Middle High German (Paul 2007: 28) considers [w] in that language to be a bilabial glide (i.e. ‘semi-vowel’ = [‘Halbvokal’]), which only shifted to [v] at a later stage. (“Das Zeichen steht mhd. noch für den bilabialen Halbvokal [w] (wie engl. w), der erst später zum labiodentalen Laut [v] wird”). At a later point in his grammar, Paul (2007: 142) writes that MHG [w] might have been the bilabial fricative (i.e. [β]), which began the shift to [v] in Late Middle High German.

[...] I argue that the two MHG sounds in question were glides (defined featurally in Section 5) and not fricatives. The most compelling evidence for the glide interpretation is that the sounds representing <w> and <j> are retained as glides in many modern German dialects. Two representative examples from a variety of Southern Bavarian (Schatz 1897) are provided in (26):

(26)
a. [wint] ‘wind’
b. [jɔxt] ‘hunt’

(Hall, Tracy Alan. 2017. Underlying and derived glides in Middle High German. Glossa: a journal of general linguistics 2(1): 54. 1–31, DOI: https://doi.org/10.5334/gjgl.170 ; page 13)

W as [v]

German "w", as you mentioned, is generally cognate to English "w" ([w]) and for this reason the corresponding Proto-Germanic phoneme is standardly reconstructed as [w]. German and English are standardly classified as further belonging to the "West Germanic" group specifically, so we can extend the reconstruction of [w] to Proto West Germanic.

I Googled and found a book, Analysis of the Scandinavian Loanwords in the Aldredian Glosses to the Lindisfarne Gospels, by Sara M. Pons Sanz, that says North and West Germanic are thought to have split around 500 AD or later. If accurate, this allows us to date the shift of [w] to [v] to some point between 500 AD and modern German.

"The Spelling of Proto-Germanic /f/ in Old High German" (Gustav Must, Language, vol. 43, no. 2, 1967, pp. 457–61. JSTOR, https://doi.org/10.2307/411546 accessed 9 Nov. 2023), has the following footnote:

The bilabial /w/ indeed existed in German probably as late as the 16th century, when V. Ickelsamer in his grammar described the sound of w as one like blowing on hot food: 'wie mã in ein heyss essen bläst' (Fechner 1882, on the eighteenth unnumbered page of 'Ein Teütsche Grammatica' by Valentinus Ickelsamer).

(page 459, footnote 3)

The consonant spelled "W" still patterns similarly to an approximant/sonorant in German phonology, and I believe I have read that in some regions of Germany a labiodental approximant pronunciation [ʋ] is used. (Wikipedia says this is more common in Southern accents.)

V as [f]: a unetymological spelling of unclear origin

In modern standard German, the letter "V" is pronounced as [f] in native words, where it corresponds to Proto-Germanic *f, which we're pretty sure was a voiceless labial fricative of some sort (either [f] or [ɸ]). I am not aware of any etymological explanation for the rather random use of word-initial "v" vs. "f" in modern German spelling; it could conceivably result from dialect mixing (although that explanation does little to clarify the distribution of the two alternative spellings).

Maybe dialect-mixing could be the cause

I have read that in some historical dialects of German, word-initial and intervocalic singleton *f has undergone lenition or voicing, becoming something like [v] or [v̊]; compare the lenition/voicing of word-initial *s to [z] that is well-known as a feature of standard German today (a number of regional accents of German have voiceless [s] in this position, however).

The Wikipedia article I linked to earlier says that there are accents of German that retain a distinction between semi-voiced or lenis f/v, from *f, and fortis f, from consonant-shifted *p. (This is similar to the constrast between intervocalic "s" and "ß" in modern standard German.)

However, this phonological information doesn't seem to be enough to determine the date of voicing/lenition of /f/ to /v~v̊/, because there is evidence that the HGCS initially resulted in geminate fricatives like /ff/ intervocalically. So voicing/lenition could have applied either before the HGCS, or after it, as long as it occured at some point before the loss of consonant length (a loss that I have heard is not complete in all modern German accents) and was restricted to affecting singleton /f/, not geminate /ff/.

Word-initial voicing of *f to [v] also occcured in at least some accents of Dutch (explaining the standard Dutch spelling of words like "vriend", which corresponds to the pronunciation with [v] that is present in some modern accents of Dutch, although others have voiceless [f]) and in some old accents of English (explaining the "v" in "vixen" and "vat"). However, I don't believe this voicing reflects a shared inherited feature of English, Dutch and German, but a regional tendency towards voicing word-initial as well as intervocalic fricatives (which is most robustly evident in the reflex of *þ as /d/ in both German and Dutch).

Evidently, this voicing process was either reversed, or did not occur for *f in accents ancestral to modern Standard German pronunciation.

Maybe spelling practices of Merovingian Latin, influenced by French sound changes, were the cause

Alternatively, Must 1967 thinks that the use of the spelling "V" for /f/ in German didn't develop as the result of voicing of the sound in German, but instead is derived from the use in Merovingian Latin of the letters F and V for the same sound in intervocalic position (a result of the sound change in Western Romance of Latin intervocalic /f/ to [v]).