endolabial

Protruded versus compressed rounding

There are two main types of vowel rounding: protrusion and compression, with various terminology. In protruded rounding, the corners of the mouth are drawn together and the lips protrude like a tube, with their inner surface visible. In compressed rounding, the corners of the mouth are drawn together, but the lips are also drawn together horizontally ("compressed") and do not protrude, with only their outer surface visible. That is, in protruded vowels airflow is directed along the inner surfaces of the lips (thus the alternate term endolabial), whereas in compressed vowels it is directed along the outer surfaces (thus exolabial – see § Terminology). Back and central rounded vowels, such as German /o/ and /u/, are typically protruded, whereas front rounded vowels such as German /ø/ and /y/ are typically compressed. Back or central compressed vowels and front protruded vowels are uncommon, and a contrast between the two types has been found to be phonemic in only one instance.