Inventory or material balance control is an important part of process control. In the literature, many rules have been proposed to help in designing such systems, but their justification is often unclear. The main contribution of this paper is to propose the more general local-consistency rule for evaluating inventory control systems. Consistency means that the steady-state mass balances (total, component, and phase) for the individual units and the overall plant are satisfied. In addition, locol consistency is a desired property, meaning that the local mass balances are satisfied with local inventory loops only.