The Great Vowel Shift was a major change in the pronunciation of the long vowels of the English language (i.e. a vowel shift), which began in the 15th century and was mostly completed in the 16th century, although it continued for some time after that.

The values of the long vowels form the main difference between the pronunciation of Middle English and Modern English. Originally, they were essentially the same as those found in Latin. However, during the Great Vowel Shift, the two highest long vowels became diphthongs, and the other five underwent an increase in tongue height and one of them came to the front.

The principal changes are roughly the following, though of course exceptions occur, the transitions were not always complete, and there were sometimes accompanying changes in orthography:-

/a:/ -> /ei/ (in e.g. make)
/e:/ -> /i:/ (in e.g. feet)
/i:/ -> /ai/ (in e.g. mice)
/o:/ -> /u:/ (in e.g. boot)
/u:/ -> /au/ (in e.g. mouse)

This means that the vowel in the English word make was originally pronounced as in modern English father, but has now become a diphthong, as it is today in standard pronunciations of British English (see Received Pronunciation); the vowel in feet was originally pronounced as a long Latin-like e sound; the vowel in mice was originally what the vowel in feet is now; the vowel in boot was originally a long Latin-like o sound; and the vowel in mouse was originally what the vowel in moose is now, but has now become a diphthong.

The Great Vowel Shift was studied by the Danish linguist Otto Jespersen (1860 - 1943).

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