The author points out certain conceptual problems concerning module coupling and then presents a theoretical model of module coupling which offers a better understanding of the concept. The solution is based essentially on the evaluation of the complexity of each inter-module connection and for all connections the complexity of an inter-module connection is determined by the complexity of data relying on a recently developed data complexity method. Couplings for program logic control and assignment are compared and their ratio is determined. A formula to compute the coupling complexity of modules in a system is proposed. The author demonstrates that the coupling model coincides with the empirical knowledge of software design. Therefore, they believe that their coupling model has a clarified theoretical basis and the metric developed can be used efficiently to guide software design.