One blurry area is in segments variously called semivowels, semiconsonants, or glides. On one side, there are vowel-like segments that are not in themselves syllabic, but form diphthongs as part of the syllable nucleus, as the i in English boil . On the other, there are approximants that behave like consonants in forming onsets, but are articulated very much like vowels, as the y in English yes .
One blurry area is in segments called semivowels, semiconsonants, or glides. On one side, there are vowel-like segments that are not syllabic themselves but form diphthongs as part of the syllable nucleus, such as the i in English "boil." On the other, there are approximant sounds that behave like consonants when forming onsets but are articulated very much like vowels, such as the y in English "yes."