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.