The previous chapters discussed in detail the electrical energy density in a solid with dielectric properties and the energy density of elastic deformation. From this, numerous physical correlations have been identified which allow the deformation and stress state of piezoelectric materials to be described in a thermodynamic context. This type of consideration leads to different systems of piezoelectric equations of state by introducing different thermodynamic potentials. A mathematically closed description of the static behaviour of multilayer piezoelectric bending transducers is based on the selection of a suitable pair of equations of state, taking into account the crystal symmetry of the piezoelectric material system PZT. In this chapter, the equations of state for the independent pair of variables (T, E) form the basis for a mathematically closed representation. In combination with the kinematics of the plane beam, the internal energy stored in the multilayer bending transducer can be derived. Finally, the principle of minimum total potential is used to derive the coupling equations between the applied generalised forces (moment M, force F, pressure p, electric voltage U) and the energetic conjugates (torsional displacement \(\varphi \) , displacement w, volume displacement V, charge displacement Q) as functions of the length coordinate x of a multilayer bending transducer. The coupling equations obtained are thus summarised in a matrix, also known as the static coupling matrix \(\boldsymbol{M}(x)\) .