In the International Phonetic Alphabet, inverted breve below is used to denote that the vowel is not syllabic.
The same diacritic is placed under iota (ι̯) to represent the Proto-Indo-European semivowel as it relates to Greek grammar; upsilon with an inverted breve (υ̯) is used alongside digamma to represent the Proto-Indo-European semivowel [REF].