The evolution of Middle English into early Modern English involved a systematic change in the pronunciation of long, stressed vowels. This is known as the Great Vowel Shift (GVS) and it happened from about the middle of the fifteenth century to about the middle of the sixteenth century. The term “Great Vowel Shift” was coined by the Danish linguist Otto Jepersen. The Great Vowel Shift is not something that happened overnight, but gradually evolved over several generations.
The Great Vowel Shift involved six vowels: all were long, stressed monophthongs -- vowels in stressed positions which were pronounced long and had a pure sound. For the non-linguist, what I have just written probably sounds like academic gobbledygook, so let’s look at some specific examples.
The vowel “i” as in “mice” is a high front vowel. In Middle English “mice” would have been pronounced “mees”.
The modern word “mouse” would have been pronounced “moos” in Middle English. It then evolved into “mah-oose” and then finally into the dipthong which we have today in “mouse”.
The vowel “e” as in “feet” is considered a mid-vowel. In Middle English “feet” would have been pronounced “fate”.
Another mid-vowel is “o” as in “do” which would have been pronounced “dough” in Middle English.
The vowel “a” is a low back vowel. The modern word “name” would have been pronounced “nahm” in Middle English.
The long open “o” which was pronounced “aw” became the long “o”. Thus the modern English word “so” would have been pronounced “saw” in Middle English.
Linguists summarize that in the Great Vowel Shift: (1) front vowels were raised and fronted; (2) back vowels were raised and retraced; and (3) high vowels were made into diphthongs.
Linguists have documented the fact that the Great Vowel Shift happened, but the intriguing question is why did it happen? In answering this question, linguists offer two non-exclusive hypotheses.
First, at the time of the Great Vowel Change there were migrations in English which brought people from the Midlands into contact with people from London. The mix of dialects coming into close contact may have resulted in social pressures to create pronunciations which would have new social status and prestige.
Another factor at this time is the loss of prestige for French and the growing prestige for English. As French lost status, there would have been social pressures to develop a new prestige for English.
For those who have difficulty with English spelling, it should be pointed out that spelling began to be standardized before the Great Vowel Change. This means that Modern English spellings tend to reflect Middle English pronunciation rather than Modern English pronunciation. The Great Vowel Change was accompanied by the loss of the pronunciation of the final “e”. Thus “made” would have been pronounced “mah-duh”.