(IPA,obsolete) Indicates a more-open articulation of a vowel or (after 1976) of a consonant as well, the latter specifically intended to distinguish an approximant from a fricative. Replaced with ⟨◌̞⟩ in 1989.
The IPA explains the modern symbol as follows: "Lip position in vowels is regarded as forming a continuum from close rounded (as for Cardinal [u]) to [close] spread (as for Cardinal [i]). The 'under-rounding' diacritic may be used to indicate a value on that continuum further from 'close rounded' than entailed by the reference value of a symbol, for instance [u̜] for a close back vowel with little rounding, and [ɛ̜] for a vowel with more spreading than the open-mid reference vowel."[1]
Unofficially, when the base letter descends past the baseline, the spacing diacritic ⟨˓⟩ may be used instead.
The old symbol was unambiguous, because at the time, a less-rounded vowel was indicated with a spacing diacritic: more-open ⟨o̜⟩ vs less-rounded ⟨o˓⟩. This usage was retired in 1989, the same year the spacing diacritic for less-rounded was retired, the combining diacritic inheriting the latter meaning. Unicode also provides a dedicated character, ⟨◌᪷⟩, for the equivalent Teuthonista more-open diacritic.
