occur. In progressive vowel harmony, the second vowel changes to match the first; in regressive harmony, the first changes to match the second.

The tendency to prefer adjacent high vowels to be identical also spreads across morpheme boundaries within a word. Adding a suffix to a word can place a /u/ and an /i/ in contact. When that happens, one of the vowels tends to assimilate by changing to match the other vowel. That kind of assimilation is called vowel harmony