Cooper (2013: 11–12, 2015: 317–320) suggests that /m/ was less sonorous in PIE than /l/, /r/, and /n/. This article discusses the evidence proposed for this analysis and puts forward some further evidence, of differing degrees of strength, from Sanskrit, Oscan, Venetic, Celtic and Greek. It concludes that there is some evidence for a lower sonority of /m/ than /l/, /n/ and /r/ in Greek and Sanskrit, but that the evidence for other languages is inconclusive. There are a number of instances in which /m/ patterns with plosives rather than the other sonorants in a number of other contexts, whose relevance to questions of sonority, however, is not clear. Overall, it is plausible that /m/ may have had a lower sonority than the other sonorants in PIE , but this is not necessarily the explanation for all its odd behaviour relative to the other sonorants in PIE and its descendant languages.