General Changes
In Volume I of William Labov's Principles of Linguistic Change, Labov outlines the principles of how languages change.
Chain Shifts
Chain shifting is when one sound changes and then another changes in response in order to take the place of that sound. For example, if [e] turns to [i], then [a] will turn to [e] to keep [e] in the language. One of the explanations for why chain shifting happens is for the language to preserve its ability to make distinctions between words by having one sound move in response to another sound, so there aren’t too many mergers. Apparently this isn’t the only explanation though. The three principles are:
- In chain shifts, long vowels rise
- In chain shifts, short vowels fall
- In chain shifts, back vowels move to the front.
An example of long vowels rising is the Great Vowel Shift in English. The word meet was once pronounced like mate [meːt], and time was pronounced teem [tiːm]. But then [tiːm] turned to the current pronunciation [tajm] so the e in meet became [iː] to take the place of the letter i. If you've looked at the phonology page on this site, you know that [iː] is made with a higher tongue position than [eː] is made with. And [iː] became [aj] because it couldn’t go any higher, and so it became a diphthong.
English vowels rising. No, I don't know what the /iy/ or /uw/ mean.
As for short vowels falling, take the two words meet and met. Once, these words contained the same vowel [e]. However, in meet the vowel was long [eː] and in met the vowel was short [e]. So while long [eː] rose to [iː], short [e] fell to our current short e in met, [ɛ].
I didn’t learn about any good examples of back vowels moving to the front, but I did read an interesting explanation for why it happens. The attested explanation is that languages have a drive for symmetry, so they want to have the same number of back vowels as they do front vowels. Unfortunately, there is more room in the front of the mouth than there is in the back, so back vowels move to the front to relieve overcrowding. I’m guessing this continues on as more back vowels are added and more move to the front, and more are added again. But I didn’t read anything about developing new back vowels, at least when it comes to chain shifts. Also, as far as I know, this isn’t the only suggested explanation.