Engineers are already familiar with the most common mechanical constitutive equation that applies for everyday metals and plastics. This equation is known as Hooke's Law and is written as:

However, since piezoelectric materials are concerned with electrical properties too, we must also consider the constitutive equation for common dielectrics:

In order to describe or model piezoelectric materials, one must have knowledge about the material's mechanical properties (compliance or stiffness), its electrical properties (permittivity), and its piezoelectric coupling properties.

For example, the subscript E on the compliance matrix s_E means that the compliance data was measured under at least a constant, and preferably a zero, electric field.

Likewise, the subscript T on the permittivity matrix e_T means that the permittivity data was measured under at least a constant, and preferably a zero, stress field.