↑ Endolabial: also called protruded, lip-pouting, horizontal lip-rounding, outrounding, and inner rounding (Trask 1996:180) Exolabial: also called compressed, pursed, vertical lip-rounding, inrounding, or outer roundingTemplat:Verify source (Trask 1996:252) Henry Sweet noted in 1890 that "the term 'inner rounding' derives from the use of the inner surfaces of the lips; the synonymous 'outrounding' derives from the forward projection of the lips. Both terms are justifiable, but their coexistence is likely to lead to serious confusion." (Trask 1996:180.)
↑ Henry Sweet (A handbook of phonetics, 1877) noted that they are less distinctive from unrounded vowels than their counterparts.
↑ Bahasa Jepun ada vokal terketap belakang (Okada 1999:118); Bahasa Sweden ada vokal terketap belakang <o> dan pasangan vokal terketap depan <u> dan terjulur depan <y> (Engstrand 1999:141); vokal bundar depan tersebut berbeza dalam ryta 'mengaum' dan ruta 'kaca tingkap' (Ladefoged & Maddieson 1996:295).
