  We discuss the symmetries of quenched QCD with Wilson fermions, starting from
its lagrangian formulation, taking into account the constraints needed for
convergence of the ghost-quark functional integral. We construct the
corresponding chiral effective lagrangian, including terms linear and quadratic
in the lattice spacing. This allows us to study the phase structure of the
quenched theory, and compare it to that in the unquenched theory. In particular
we study whether there may be an Aoki phase (with parity and flavor
spontaneously broken) or a first order transition line (with no symmetry
breaking but meson masses proportional to the lattice spacing), which are the
two possibilities in the unquenched theory. The presence of such phase
structure, and the concomitant long-range correlations, has important
implications for numerical studies using both quenched and dynamical overlap
and domain-wall fermions. We argue that the phase structure is qualitatively
the same as in the unquenched theory, with the choice between the two
possibilities depending on the sign of a parameter in the low-energy effective
theory.
