The 'foot-strut' split
Between the 12th Century and the 18th Century, English vowel sounds changed dramatically, in what's known as the Great Vowel Shift. Long vowel sounds like "ee"s and "oo"s changed pronunciation – but not in all words, and not in all regions.
The reason for the shift is unknown, but it had a big effect on how English was spoken. And while the vowel sounds changed, the spellings of words tended to remain the same – which meant the spellings no longer consistently reflected the sound of words. "It has a lot to answer for when it comes to why English spelling is so complicated, and why there's so many different ways to pronounce an 'a' or an 'e,'" says Holly Dann, pronunciations editor at the Oxford English Dictionary (OED).
The shift didn't happen everywhere all at once. In some areas of the country older pronunciations were preserved, says Dann.
