The polarization « switching » (deformation-) of a ferroic crystal directly from a monodomain situation to its opposite, can only happen by application of very strong applied conjugated fields. In other cases, the reversal is function of the domains. Two extreme situations are studied: domain walls in lateral displacement, and domains in the shape of slats, of needles with longitudinal motions. Then the well-known variations of the macroscopic quantities (hysteresis cycles, switching currents, emission pulses) are related to microscopic phenomena: nuclei of domains, interactions, domain freezing, avalanches …The two simple studied situations highlight many results obtained in more complex cases in terms of symmetry and experimental conditions.