Great Vowel Shift

the complex series of sound developments (c. 1400 to c. 1750) which changed the vowel system of Middle English into that of Modern English: Middle English long high vowels (ē and o̵̅o̅) changed to Modern English diphthongs (ī and o̵u), and long mid and low vowels (ā, ō, and ä) were raised in their tongue positions to the Modern English sounds (ē, o̵̅o̅, and ā), but the orthography remained largely the same