It is shown that softening the acoustic mode, which brings about the occurrence of a proper ferroelastic phase transition in some crystals, is substantially due to the “kinematic” anharmonicity. The latter arises at the transition from the natural curvilinear coordinates of interatomic separation to the Cartesian atomic displacements. It is shown that the internal pressure may be represented as driving force for a proper ferroelastic phase transition induced by cation exchange. It has been found that the internal stress tensor, generated by the cation exchange, is of a more complicated nature than the tensor of the external stress. This difference comes from a specific coupling of the substituting cations with local, microscopic displacements of the neighbouring atoms inside the unit cell. It becomes evident why in a number of experiments a significant difference in the action of internal and external pressures on the crystal structure is observed and, also, why the internal pressure causes the greater anisotropy than the external one. We have obtained an equation for the crystals, in which the softening of the acoustic modes is caused only by the kinematic anharmonicity, with whose help we can predict either the external pressure of the ferroelastic phase transition to a triclinic phase if the initial elastic moduli are known or the internal pressure induced by cation exchange for the isotropic case.