The relation between the notion of crystalline symmetry and characteristic
time intervals when this symmetry could be observed is analyzed. Several time
scales are shown to exist for a system of interacting particles. It is only
when the observation time is much larger than the mesoscopic fluctuation time,
the notion of crystalline symmetry becomes physically meaningful. The ideas are
concretized by a two-phase lattice model.Comment: Latex file, 10 pages, no figures. Invited repor