Great Vowel Shift in English. Long high vowels /iː/ and /uː/ diphthongized into /aɪ/ and /aʊ/ respectively, illustrating a major shift from monophthongal to diphthongal pronunciation impacting words like "mice" and "house". The shift affected the pronunciation of approximately 15% of English vowels creating a notable divergence from its earlier pronunciation, influencing dialects and modern pronunciation standards across many English speaking regions.