>BTW what are the rules for -dh- in inlaut in Latin?
The outcome of medial *dh is normally b or d, but f can also be found
(often claimed to be Sabellian or Faliscan dialect forms). d is the regular
outcome; b is sometimes found near u, l, or r.
e.g. viduus, medius; but verbum, iubeo, stabulum
< *widHew-, medHio- ; werdHo- , ioudH-eie-, sth2-dHlo-
The processes are unclear. There could be:
general devoicing - spirantisation, then voicing - hardening
medially,
or
general spirantisation - devoicing initially, hardening medially.
Initially the outcome is a spirant /f/, but before r or l we get a stop
consonant (instead of a spirant) even initially.
Peter