  We derive a set of symmetry relations for the three-neutrino mixing angles,
including the MSW matter effect. Though interesting in their own right, these
relations are used to choose the physical region of the mixing angles such that
oscillations are parameterized completely and uniquely. We propose that the
preferred way of setting the bounds on the mixing angles should be $\theta_{12}
\in [0,\pi/2]$, $\theta_{13} \in [-\pi/2,\pi/2]$, $\theta_{23}\in [0,\pi/2]$,
and $\delta \in [0,\pi)$. No CP violation then results simply from setting
$\delta=0$. In the presence of the MSW effect, this choice of bounds is a new
result. Since the size of the asymmetry about $\theta_{13} = 0$ is dependent on
the details of the data analysis and is a part of the results of the analysis,
we argue that the negative values of $\theta_{13}$ should not be ignored.
