In order to visualise the phase diagram of a quaternary system under constant temperature and pressure one would need three dimensions. However, in some cases much information can be presented in a quasi-ternary section. This is the case for the Si-Al-O-N system where all the condensed phases fall in or very close to the section defined by the corners Si3N4, SiO2, Al2O3 and AIN as if all the phases were ionic and composed of Si+4, Al+3, O-2 and N-3, which is not quite true. It has been possible to model the properties of the condensed phases using models which restrict their existence to the section mentioned. The modelling will now be described briefly and it will be demonstrated that it was possible to extrapolate the properties of the ternary side systems and use the meagre information available from the quaternary system in order to predict the phase relations [91Hill2].