Parameters' domain in three flavour neutrino oscillations

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.