  The phases of quantum interference effects in charged lepton production by
neutrinos (neutrino oscillations) following pion decay at rest and in flight,
muon decay and nuclear $\beta-$decay at rest as well as for `muon oscillations'
following pion decay at rest and in flight are calculated. The same phase is
found for neutrino and muon oscillations following pion decay at rest. The
results found disagree with the conventionally used value: $\phi_{12} = \Delta
m^2 L/(2P)$. Differences to previous treatments are briefly commented on
