The spontaneous polarization in ferroelectric crystals is a polar vector so that there must not be any symmetry operator in the point group of a ferroelectric crystal that changes the direction of the polarization. On the other hand, the point group of the crystal must not contain the inversion. Therefore, there are only 10 crystal classes that allow a macroscopic spontaneous polarization, namely 1, 2, m, mm2, 4, 4mm, 3, 3m, 6 and 6mm, and thus the ferroelectricity can only be induced by the phase transitions to one of these crystal classes.