On October 25th in the year 1400 the great poet Geoffrey Chaucer died. Linguists use his passing to mark the beginning of evolution from Middle English to Modern English. One of the most significant linguistic events of that transition was the Great Vowel Shift, a dramatic change in the pronunciation of the long vowels that took nearly 200 years to run its course.

No one is sure what prompted the modification in the pronunciation of English vowels, but everyone agrees those changes had a profound effect on the language. If you have read Chaucer's original works, you know that they are extremely difficult to interpret. But Shakespeare's texts, written less than 200 years later, are easy for modern-English speakers to understand.

Scholars think the Great Vowel Shift began with a change in just one vowel sound (although there is disagreement about which one). After one pronunciation changed, so the theory goes, the pronunciations of the others gradually shifted to maintain distinctions between vowels. The transitions also generated new diphthongs, two-element speech sounds that start with the tongue in one position and end with it in a different position (as in the "ou" of "out").

Unfortunately, the Great Vowel Shift coincided with the rise of the printing press in England. Just as new printing technology was solidifying spelling, the pronunciation of the vowels was shifting around. That contributes to the disjunct between spelling and pronunciation that we suffer today.