Great Vowel Shift
The Great Vowel Shift is the name given to a series of major changes to English that started at the beginning of the 15th century and which continued for over 200 years. The changes affected the spelling and pronunciation of long vowels (the short vowels were not affected). It explains why we say, for example, child with a long vowel, /aɪ/, but children with a short one.
In Chaucer's time, the "a" in name was pronounced [aː] as in arm, but by Shakespeare's time it was being pronounced [ɛː] like the "e" in them (but longer), until, by the 18th century it has finally become the [eɪ] we know today.
- Crystal, D. Spell It Out: The Singular Story of English Spelling Profile Books ISBN 978-184668567 5