  We argue that the appropriate variable to study a non trivial geometry
dependence of the Casimir force is the lateral component of the Casimir force,
which we evaluate between two corrugated metallic plates outside the validity
of the Proximity Force Approximation (PFA). The metallic plates are described
by the plasma model, with arbitrary values for the plasma wavelength, the plate
separation and the corrugation period, the corrugation amplitude remaining the
smallest length scale. Our analysis shows that in realistic experimental
situations the Proximity Force Approximation overestimates the force by up to
30%.
