In Vowels^x, Consonants^x, Approximants^x

I do not think it necessarily is.If we start with a definition that a vowel is a sound articulated without any obstruction of the vocal tract and that a consonant is a sound that is articulated with an obstruction, then we do not have any difficulty deciding that /a/ is a vowel and /k/ is a...

I suspect what the question is trying to elicit is something like this: We tend to think that there is a hard and fast distinction between vowels and consonants. There are however some sounds (nasals and approximants) which (like vowels) do not involve any friction of the organs of speech, but...

Why not consonants? The class of consonants has nasals and approximants, but not vowels. I suspect there is more assumed in the question than is actually stated. I suspect the word only has been left out: Is there a natural class that has only nasals and approximants but not...

Yes, I do sound a little like John Wayne, pardner. No need to get in a twist, King-- my tongue is obviously doing something back there: it is all tensed up, controlling the air flow and who knows what-all. The discussion, however, is intended to get foreign students to differentiate their /r/...