Vowel harmony in Turkish and other Turkic languages has "multiple features" — in some cases, agreement between vowels is required with respect to more than one distinctive feature. One of those features is called "backness" or what is more formally called palatal harmony. While all Turkic vowels agree in "backness" (palatal harmony), some high vowels may also agree in "roundness" (labial harmony). For example, the root vowel in the word , meaning face, is rounded, but the nominative plural suffix only agrees with the root vowel in backness, not roundness (). The genitive singular suffix -ün, as in agrees with the root vowel in respect to both backness and roundness.
