The main physical characteristics of potassium titanyl phosphate-type ferroelectrics AMOXO(4) are related to crystal structure parameters such as the basic and interstitial positions of the A atoms and the MO6 octahedra distortions. On these grounds, a new understanding of the structural origins of the high ionic conductivity in silicates and germanates of the KTiOPO4 family is reached. The optical nonlinearity and T-c behaviour in solid solutions with niobium starting from KSbOGeO4 and KTaOGeO4 is also discussed.