Such pronunciation changes are a regular feature of language.
Sometimes they occur with the speed of centuries, sometimes with seemingly
hell-for-leather haste. They appear from time to time in all languages for reasons
that no one really understands.
German had one not long after the departure of the Angles and Saxons to
Britain, which resulted in the division of German into High and Low varieties. In
the German shift, northern speakers came to places's where before they had put
t's, and to put f's where previously they had employed p's. These changes were
of course too late to affect English, and thus explain the differences in many
modern English and German words, such as water and wasser and open and
offen. Such changes are by no means unique to English or even the Germanic
languages.
Latin underwent a prolonged series of changes. In the fourth century, to take one
example, the Latin centum (hundred) began to be pronounced in various ways—
a fact reflected in the modern French cent, "sent," Spanish ciento, "thiento," and
Italian cento, "chento." By such means did the Romance languages grow.
In England the Great Vowel Shift, as it is generally and somewhat misleadingly
called, happened later, roughly around the time of Chaucer. Textbook
discussions of the shift can sometimes leave us with the impression that people
pronounced their vowels in one way up to a certain date and then suddenly, as if
on a whim, began pronouncing them in an altogether different way. But of
course it was never as simple as that. Many of the pronunciation changes
reflected changes that had begun centuries before in the time of King Alfred and
some of them are not complete to this day. (Shove and move may one day be
pronounced in the same way; it would make sense.) So, although it is true to say
that these constituted some of the most sudden and dramatic changes English
had ever undergone, we should not lose sight of the fact that we are talking about
a period that spanned, even at its most rapid, a couple of generations. When Chaucer died in 1400 , people still pronounced the e on the end of words.
One hundred years later not only had it become silent, but scholars were evidently
unaware that it ever had been pronounced. In short, changes that seem to history
to have been almost breathtakingly sudden will often have gone unnoticed by
those who lived through them.
No one knows why this vowel shift happened. As Charlton Laird has succinctly
put it: "For some reason, Englishmen started shoving tense vowels forward in
their mouths. Then they stopped. And they have remained stopped. Nobody
knows why they started or why they stopped." For whatever reasons, in a
relatively short period the long vowel sounds of English (or tense vowels as
Laird called them) changed their values in a fundamental and seemingly
systematic way, each of them moving forward and upward in the mouth. There
was evidently a chain reaction in which each shifting vowel pushed the next one
forward: The "o" sound of spot became the "a" sound of spat, while spat became
speet, speet became spate, and so on. The "aw" sound of law became the "oh"
sound of close, which in turn became the "oo" sound of food. Chaucer's lyf,
pronounced "leef," became Shakespeare's life, pronounced "lafe," became our
life. Not all vowels were affected. The short e of bed and the short i of sit, for
instance, were unmoved, so that we pronounce those words today just as the
Venerable Bede said them 1,200 years ago.
There were other changes as well—most notably the loss of the Old English
sound x, the throat-clearing sound of the ch in the Scottish loch or the German
ach. The loss of this sound from English meant that others rushed to fill the
vacuum, as in the Old English word burh (place) which became variously burgh
as in Edinburgh, borough as in Gainsborough, brough as in Middlesbrough, and
bury as in Canterbury.
Before the shift house was pronounced "hoose" (it still is in Scotland), mode was
pronounced "mood," and home rhymed with "gloom," which is why Domesday
Book is pronounced and sometimes called Doomsday. (The word has nothing to
do with the modern word doom, incidentally. It is related to the domes-in
domestic.) But as with most things, shifting vowel sounds were somewhat hit or
miss, often because regional variations disrupted the pattern. This is most
notably demonstrated with the "oo" sound. In Chaucer's day in London, all
double o words were pronounced to rhyme with the modern word food. But once
the pattern was broken, all kinds of other variations took hold, giving us such anomalies as blood, stood, good, flood, and so on. Most of these words were
pronounced in different ways by different people from different places until they
gradually settled into their modern forms, although some have never truly
settled, such as roof and goof, which some people rhyme with goof and others
pronounce with the sound in foot. A similar drift with "ove" accounts for the
different sounds of shove, move, and hove.
Since obviously there is no one around who heard English as it was spoken in
the time of Chaucer and Caxton, how do we know all this? The answer is that for
the most part we cannot know for sure.
Most of it is based on supposition. But scholars can get a good idea of what
English must have sounded like by looking at the rhymes and rhythms of historic
verse and by examining the way words were spelled in letters and other snatches
of informal writing. In this respect we owe a huge debt to bad spellers. It is from
misspellings in letters of the seventeenth, eighteenth, and nineteenth centuries
that we can be pretty certain that boiled was pronounced byled, that join was
gine, that merchant was marchant, and so on.
From the misspellings of Queen Elizabeth we know that work was once
pronounced "wark," person was "parson," heard was "hard," and defer
was"defar," at least at court.
In the same period, short vowels were often used interchangeably, so that not was sometimes
written nat and when sometimes appeared as whan. Relics of this variability
include strap and strop, taffy and toffy, God and gad.