The Great Vowel Shift refers to the 15th century change in pronunciation of long that occurred in England. After the Great Vowel Shift, vowel pronunciation shifted up one place. So, for example the i in Middle English had a long e sound, as in the word sweet. After the Great Vowel Shift, one sees the long i sound pronounced as it is currently pronounced, such as in the word night.

The Great Vowel Shift is something of a mystery, and linguists have been unable to account for why it took place. It was first identified and studied by Otto Jesperson, a linguist from Denmark, during the late 19th and early 20th centuries.

Most linguists agree that the Great Vowel Shift did not occur all at once, which accounts for the creative spellings of many English words. Some printers might still have employed an earlier vowel pronunciation in spelling words, thus making English one of the most challenging languages to spell, because so many exceptions to spelling rules exist.

Some linguists account for the Great Vowel Shift by suggesting that England’s rule by the French led to disenchantment with French pronunciation of vowels, which is a similar pronunciation to that of Middle English. To distance themselves from prior French occupation and rule, the English ruling class may have deliberately changed pronunciation to reflect that theirs was a language different from French. This then filtered down to the lower classes.

One interesting theory on the reason for the Great Vowel Shift is that England may have had several influential people with speech impediments. Such mispronunciations might be copied in deference to someone of high enough rank. This theory is not endorsed by many, but does show linguists attempting to consider all possible explanations for the change.

The theories regarding the Great Vowel Shift are merely conjectures. Most linguists lean toward the former theory above as a reasonable explanation for the change in pronunciation. The key pronunciation features of the Great Vowel Shift are the following:

• Middle English (ME) a is pronounced as the a in father. Early modern English (EME) pronounces the long a as in gate.
• ME pronounces the long e as the long a in gate. EME pronounces the long e as the e in tweet.
• ME pronounces the long i as the e in tweet. EME pronounces the long i as the i in light.
• ME pronounces the long o as the o in tool. EME pronounces the long o as the o in goal.

ME scholars suggest that no higher long u pronunciation exists. The ou as in current day, would have given the “ow” sound, as in the word louse. EME pronounces the u as long o in ME. Long u pronunciation in EME is as the long o of tool or the long u of lute.

There are naturally pronunciation exceptions, such as the words tool and lute. Why words with the same essential sound are spelled different suggests that the Great Vowel Shift was certainly not uniform, and did occur over time. Theoretically, tool could reasonably be spelled tule, as is mule. Whatever the theory, we must look to the Great Vowel Shift as the forebear of modern English pronunciation, and also as to why we spell so many words in ways that make little sense from a phonetic standpoint.