【正确答案】The great vowel shift is the systematic and regular change in the vowel sounds．It occurred at the end of the Middle English period，approximately between 1400 and 1600．It is a major change in the history of English that led to one of the major discrepancies between the phonemic representations of words and morphemes，that is，between the pronunciation and the spelling system of Modern English．The great vowel shift involves seven long，or tense vowels of Middle English．Some examples are given below in phonetic transcription．