What Are the Differences Between Front and Back Vowels?

Modern linguists have studied these vowel changes and they've been able to identify many of the rules which marked the changing sounds of vowels over the centuries. A lot of this early work was done in Germany and the German word for these vowel changes is oomle out. Now some English linguists also use that term but the more common term in English is mutation. And since a lot of vowels shifted to a vowel sound represented by the letter I, this type of vowel shift is often called I mutation.