1. Old English and Middle English
Already in the Old English period, the OE vowels /y/ and /y:/ had unrounded in different ways in different dialects, according to the following scheme.
|/y/, /y:/||> /i/, /i:/ in the North and the East Midlands|
|> /e/, /e:/ in the South|
|> /y/, /y:/ in the West Midlands|
By the end of the Middle English period, all dialects had /I/, /i/. But dialectical survivals account for some of the apparently "irregular" developments of the vowel and some of the peculiarities of English spelling. Merry, for example, from OE myrige, retains both the Southern pronunciation and the Southern spelling. PDE busy, on the other hand, from OE bysig, exhibits the Eastern pronunciation but the Western spelling u, the normal spelling of the rounded vowel in ME.
2. Early Modern English
In Early Modern English, ME /U/ centered and unrounded to produce the stressed version of EME /'/. In general (though with some exceptions), this unrounding did not occur if /U/ was preceded by a labial consonant (e.g., /p/, /f/, /b/) and followed by /l/, /s</, or /c</.