Does anyone know how you pronounce the root vowel of the word cometh in ME and EModE? What is this particular sound change called?
In Middle English it was /u/ — en.wiktionary.org/wiki/comen#Middle_English. The letter
o is written there for practical reasons: near
v made up of vertical strokes the letter
u also made up of vertical strokes is hard to discern, many vertical strokes in a row are ambiguous when hand-written.
And, irrespective of its spelling, ME /u/ > ModE /ʌ/, like in ‘but’ or ‘cup’. In ‘some’ (< ‘summe’), ‘love’ (< ‘luve’), ‘wonder’, ‘month’, etc. the original letter
u was replaced by
o because of the reason I mentioned above. Here's an article about this substitution:
The Clever Monks and the Lazy O: Why O sometimes says /ŭ/.
This change of /u/ > /ʌ/ is a part of what is called “FOOT–STRUT split”, the split of Middle English short /u/ into two distinct phonemes: /ʊ/ (as in foot) and /ʌ/ (as in strut).