A restructuring of the vowel system of Early Modern English, which took place from the mid 14th century to the beginning of the 16th century. All the vowels of the long vowel subsystem raised by one degree, open to open-mid, open-mid to close-mid, close-mid to close. The two close vowels i and u became diphthongs, ultimately aɪ and aʊ respectively.

Some examples:

• Middle English se: became siː see

See height for an explanation of the terms above.