^Protrusion is also called endolabial, lip-pouting, horizontal lip-rounding, outrounding, or inner rounding （Trask 1996，p.180）.
^Compression is also called exolabial, pursed, vertical lip-rounding, inrounding, or outer rounding （Trask 1996，p.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，p.180）
^Sweet (1877) noted that they are less distinctive from unrounded vowels than their counterparts.