This paper deals with the construction and use of a simple linear supply and demand model. The main advantages of using such a model is firstly that everything is formulated and understood in the fundamental concepts of economic theory, and secondly that the model is easy to use in simulating various scenarios. It is not an alternative to larger and more sophisticated models, but can sometimes be a useful complement. It is formulated so that it can give simple and robust answers what will happen to production, consumption and trade if demand and supply change. It is easy to use regardless of the number of regions involved.