Great Vowel Shift

The main difference between the pronunciation of Middle English and Modern English lies in the values of the long vowels, which originally were essentially the same as those found in Latin. This change is called the Great Vowel Shift. Most of the change took place in the 1400s and 1500s, although it continued for some time after that.

The two highest vowels became diphtongs the other five long vowels underwent an increase in tongue height and one of them came to the front.

The principal changes are roughly the following, though of course exceptions occur, the transitions were not always complete, and there were sometimes accompanying changes in orthography:

 /a/ -> /e/
 /e/ -> /i/
 /i/ -> /ai/
 /o/ -> /u/
 /u/ -> /au/