  We reveal the origin of the CP violation required by the leptogenesis in
variously popular seesaw models. Especially we clarify that in a pure
type-I/III seesaw with two fermion singlets/triplets, a combined type-I+III
seesaw with one fermion singlet and one fermion triplet, or a combined
type-I/III+II seesaw with one fermion singlet/triplet and one Higgs triplet,
the CP violation for the leptogenesis can exactly come from the imaginary part
of the neutrino mass matrix in a special basis where the Yukawa couplings
involving one fermion singlet/triplet are allowed to get rid of any CP phases.
We also generalize our findings as a very good approximation when these seesaw
scenarios are extended by more fermion singlets/triplets and Higgs triplets
while the leptogenesis is realized by the decays of the lightest fermion
singlet/triplet.
