Learning about how language changes overtime is fascinating.  An excerpt from Lane Greene essay over at Aeon.

“There is something odd about the vowels of English. Have you ever noticed that every language in Europe seems to use the letter A the same way? From latte to lager to tapas, Italian, German and Spanish all seem to use it for the ah sound. And at some level, this seems natural; if you learn frango is ‘chicken’ in Portuguese, you will probably know to pronounce it with an ah, not an ay. How, then, did English get A to sound like it does in plate, name, face and so on?

Look around the other ‘long’ vowels in English, and they seem out of whack in similar ways. The letter I has an ee sound from Nice to Nizhni Novgorod; why does it have the sound it does in English write and ride? And why do two Os yield the sound they do in boot and food?

The answer is the Great Vowel Shift. From the middle English period and continuing into the early modern era, the entire set of English long vowels underwent a radical disruption. Meet used to be pronounced a bit like modern mate. Boot used to sound like boat. (But both vowels were monophthongs, not diphthongs; the modern long A is really pronounced like ay-ee said quickly, but the vowel in medieval meet was a pure single vowel.)

During the Great Vowel Shift, ee and oo started to move towards the sounds they have today. Nobody knows why. It’s likely that some people noticed at the time and groused about it. In any case, there was really a problem: now ee was too close to the vowel in time, which in that era was pronounced tee-muh. And oo was too close to the vowel in house, which was then pronounced hoose.

Speakers didn’t passively accept the confusion. What happened next shows the genius of what economists call spontaneous order. In response to their new pushy neighbours in the vowel space, the vowels in time and house started to change, too, becoming something like tuh-eem and huh-oos. Other changes prompted yet more changes, too: the vowel in mate – then pronounced mah-tuh – moved towards the sound of the modern vowel in cat. That made it a little too close to meat, which was pronounced like a drawn-out version of the modern met. So the vowel in meat changed too.

Throughout the system, vowels were on the move. Nobody in a 15th-century tavern (men carried knives back then) wants to confuse meet, meat and mate. So they responded to a potentially damaging change by changing something else. A few vowels ended up merging. So meet and meat became homophones. But mostly the system just settled down with each vowel in a new place. It was the Great Vowel Shift, not the Great Vowel Pile-Up.

Such shifts are common enough that they have earned a name: ‘chain shifts’. These are what happens when one change prompts another, which in turn prompts yet another, and so on, until the language arrives at a new equilibrium.”