Anglic sound changes

[bradrn]

Is there any reason Middle English /iː uː/ are listed as having become [ei ou]? I thought the conventional understanding was that the shift was [iː uː] > [əɪ əu] > [aɪ aʊ] (for me and some others > [ɑɪ æʊ]).

The Wikipedia article on the Great Vowel Shift mentions both the [əi]/[əu] and [ei]/[ou] developments as possibilities and cites sources supporting each. I think the outcomes in regional speech, at least in southern England, suggest the former, and that's also more consistent with how they interacted with the ME diphthongs; in particular /iː/ merged with /ui/ (as mentioned earlier in this thread) but not with /ei/ (or however you want to transcribe the ancestor of the EModE vowel in main, wait etc.).

This is interesting. Looking at Wikipedia’s references, the only one I can get access to is Lass’s Cambridge History of the English Language, which advocates [ei]/[ou]; the book in general looks exceedingly useful, but in this case it leaves me with no way to assess what’s correct here.

…or should I maybe not bother with the decision, and indicate both alternatives as having been suggested in the literature? That might be a better option in general.

I'm not sure that the origin of the long vowel in father is actually understood: when it developed or why.

I don’t believe it is. What does everyone suggest I do about cases like this?

[anteallach]

I would give both alternatives. Indeed, it's quite possible that both may have happened in different parts of the country.