:Vowel harmony:

A constraint on the co-occurrence of vowel sounds within some domain, usually the word. There are many different vowel harmony systems in the world’s languages. A fairly common pattern involves (1) lip rounding: all the vowels in a word must be rounded or else they must all be unrounded and (2) backness: front and back vowels are not allowed to occur together in the same word. Examples of vowel harmony languages are Hungarian, Kirghiz and Turkish.