A ferroic crystal has two stable energy situations with opposite values of an extensive quantity (polarization for ferroelectricity, deformation for ferroelasticity) and the possibility of passing from one to the other under the action of a conjugated external intensive quantity. The variety of transitions observed is illustrated by some examples of changes in symmetry. The Landau model and the order parameter are introduced with a theoretical crystal of infinite dimensions. Even in the case (very theoretical) of the same value for the order parameter in all the sample (monodomain), it is possible to predict the effects of the sample shape and dimensions and the importance of the experimental conditions.