The linear piezoelectric equations of state with strain, electric displacement, and entropy, as independent variables, are derived from the principle of conservation of energy. These thirteen tensor equations are then specialized to the adiabatic case and used as the starting point for the development of all possible linear piezoelectric equations of state using in turn as independent variables, stress and electric field, strain and polarization, and stress and polarization. This process gives as a by‐product the set of relations among the elastic, electric, and piezoelectric coefficients for the various pairs of independent parameters and emphasizes the frequently overlooked fact that the piezoelectric equations of state are mutually convertible. M.k.s. units and the I.R.E. notation are used throughout the discussion and a table is given which compares the I.R.E. notation with the former notations of Cady and Mason.