Generative phonologists usually take high vowels like [i,u] and glides like [j,w] to be identical (respectively) in terms of distinctive features. The main argument for this assumption comes from theoretical economy: since syllable theory independently provides us with a means of distinguishing between vowels and glides – as syllable nuclei and margins respectively – there is no need for a featural distinction. This paper shows that in fact vowels must be featurally distinct from glides. Evidence for this comes from phonological processes that distinguish between the segment types, in a way that specifically diagnoses a difference of constriction degree. I further show that the vowel vs. glide distinction is part of a larger hierarchy of segmental distinctions based on constriction degree, as indicated by cross-linguistic generalizations. The implications of these conclusions for cases of high vowel vs. glide contrast are explored.