  It has been proposed that partially quenched chiral perturbation theory can
be used together with partially quenched lattice QCD to determine the
low-energy constants of real QCD. We point out a complication of this approach
and show how it can be resolved. Partial quenching changes the chiral symmetry
group and introduces a new independent four-derivative operator. We explore the
effects of this new operator. We describe tree-level scattering processes to
which it contributes, and show how they can be used, in principle, to
deterimine its unknown coefficient. We also calculate its lowest-order
(one-loop) contribution to charged meson masses and decay constants. In
addition, we present the form of the analytic next-to-next-to-leading order
meson mass and decay constant corrections, which are needed for lattice fits.
