  The standard (Euclidean) action principle for the gravitational field implies
that for spacetimes with black hole topology, the opening angle at the horizon
and the horizon area are canonical conjugates. It is shown that the opening
angle bears the same relation to the horizon area that the time separation
bears to the mass at infinity. The dependence of the wave function on this new
degree of freedom is governed by an extended Wheeler-DeWitt equation. Summing
over all horizon areas yields the black hole entropy.
