As will be discussed in Section 2, it seems likely that this is a typologically unusual sonority hierarchy, and it is not clear whether Cooper really means to assert that /w/ and /j/ are equally as sonorous as the liquids (and /n/). This is to say that the vowels are more sonorous than the glides, liquids and /n/, which in turn are more sonorous than /m/, which is more sonorous than the obstruents. On the basis of this observation, and with the addition of some other evidence, Cooper (2013: 11-12, 2015: 317-320 already raised as a possibility by Kobayashi 2004: 91-96) suggests that /m/ is lower in sonority than /n/, and sets up the following sonority hierarchy for PIE: vowels > /w/, /j/, /l/, /r/, /n/ > /m/ > obstruents. 1 However, as pointed out by Schindler (1977: 56–57), there are a number of phonological or morphological categories in which /m/ does not act as would be expected of the other sonorants, which will be discussed in Sections 3 and 4. Thus, a root of the structure TReRT, where T represents any obstruent and R any sonorant, is permitted, but not RTeTR.
Thus, for example, they have in common the trait of being able to act as the nucleus of a syllable and they appear nearer the nucleus of the PIE root than an obstruent, if present. In general, in the relevant contexts, these tend to pattern as a group. Proto-Indo-European had a class of sonorant consonants consisting of two liquids /l/ and /r/, two nasals /m/ and /n/ and two glides /w/ and /j/.