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.