  We consider analytically the domain of the three mixing angles $\Theta_{ij}$
and the CP phase $\delta$ for three flavour neutrino oscillations both in
vacuum and matter. Similarly to the quark sector, it is necessary and
sufficient to let all the mixing angles $\Theta_{12},\Theta_{13},\Theta_{23}$
and $\delta$ be in the range $<0,\frac{\pi}{2}>$ and $0 \leq \delta < 2 \pi$,
respectively. To exploit the full range of $\delta$ will be important in future
when more precise fits are possible, even without CP violation measurements.
With the above assumption on the angles we can restrict ourselves to the
natural order of masses $m_1<m_2<m_3$. Considerations of the mass schemes with
some negative $\delta m^2$'s, though for some reasons useful, are not necessary
from the point of view of neutrino oscillation parametrization and cause double
counting only. These conclusions are independent of matter effects.
