The electromechanical equations of state are written in a number of forms, extending from Voigt's formulation to the recently introduced formulation withgandhas the piezoelectric constants. Those equations that are most appropriate in the theoretical treatment of the two chief types of crystal transducer are pointed out. A detailed treatment of the thickness‐vibration transducer is then given, leading to expressions for the electrical characteristics and for the acoustic vibrational amplitude and intensity. Some special cases are also considered.