5. The Great Vowel Shift 
- mismatch between spelling and pronunciation: begun in ME, spelling fixed by early XVI century, but the sound kept changing).
- change of quality of all long vowels (stressed syllables) - standardization of the language: synthetic -> analytic - most significant of all phonological developments in English - change in pronunciaion of verb tense
- GRAMLEY STEPHAN. The history of English: an introduction. New York: Routledge, 2012, xxv, 123-154 pp.