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A formal model for describing and evaluating visibility control mechanisms is introduced. The model reflects a general view of visibility in which the concepts of requisition of access and provision of access are distinguished. This model provides a means for characterizing and reasoning about the various properties of visibility control mechanisms. Specifically, the notion of preciseness is defined. The utility of the model is illustrated by using it to evaluate and compare the relative strengths and weaknesses, with respect to preciseness, of the visibility control mechanisms found in Algol 60, Ada, Gypsy, and an approach called PIC, which specifically addresses the concerns of visibility control in large software systems.