In phonology, vowel harmony is a phonological rule in which the vowels of a given domain – typically a phonological word – must share certain distinctive features. Vowel harmony is typically long distance, meaning that the affected vowels do not need to be immediately adjacent, and there can be intervening segments between the affected vowels. Generally one vowel will trigger a shift in other vowels, either progressively or regressively, within the domain, such that the affected vowels match the relevant feature of the trigger vowel. Common phonological features that define the natural classes of vowels involved in vowel harmony include vowel backness, vowel height, nasalization, roundedness, and advanced and retracted tongue root.

In phonetics, advanced tongue root (ATR) and retracted tongue root (RTR) are contrasting states of the root of the tongue during the pronunciation of vowels in some languages, especially in Western and Eastern Africa, but also in Kazakh and Mongolian. ATR vs RTR was once suggested to be the basis for the distinction between tense and lax vowels in European languages such as German, but that no longer seems tenable.