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.