  We present the results of the thermodynamic theory of piezoelectric
ferroelectrics and of the phenomenological theory of thermodynamic
characteristics of the KH$_{2}$PO$_{4}$ type ferroelectrics. Available
experimental data for the dielectric, piezoelectric, and elastic
characteristics of the KH$_{2}$PO$_{4}$ type ferroelectrics and
NH$_4$H$_{2}$PO$_{4}$ type antiferroelectrics are described using the proposed
microscopic theory of these crystals. Using the results of the thermodynamic
theory and experimental data we calculate the dielectric, piezoelectric, and
elastic characteristics of the K$_{1-x}$(NH$_4$)$_x$H$_2$PO$_4$ type systems.
