Great Vowel Shift
Definition - A major change in English long-vowel pronunciation that took place in southern England between 1200 CE and 1600 CE. No one knows why it happened, but it marks the change from Middle to Modern English.
The phenomenon was first studied by the Danish linguist Otto Jespersen (1860–1943), who coined the term.
Note: Because the English spelling system became standardized during the Great Vowel Shift, it doesn't match the vowel system that exists now. This is why modern English spelling seems so irrational.

Example -

(1) Middle English hoose becomes Modern English house
(2) Middle English coo becomes Modern English cow

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