Paul Bennett wrote: > Maybe the process of (Intervocalic Voiced) Stop -> Fricative -> Glide -> > Zero ? > > Maybe (Voiced) Aspirated -> Breathy -> Schwa glide -> Schwa assimilation ? > > Both of those are plausible to me (after only a second or five thinking > about it), although I cannot speak for the actual processes involved in > PU/PFU. > > Paul Monophthongalization of diphthongs is another possible source. In Japanese, for example, there was a sound change /eu/ -> /jo:/ (presumably thru an intermediate /jou/), and /iu/ -> /ju:/. Loss of intervocalic /k/ was another source, e.g., the suffix /Siki/ -> /Si:/. Loss of coda consonants can be a source as well, e.g., Latin had changes like /ans/ -> /a:s/. However, I also know nothing about PU myself, either.