- 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.
- E.g. ⟨ɨᵝ⟩ in Flemming (2002:83).
- Deibler 1992.
- Deibler, Ellis, Alekano Organised Phonology Data, 1992