Each object­oriented programming language proposes various grouping mechanisms to bundle interacting classes (i.e., packages, modules, selector namespaces, etc). To understand this diversity and to compare the different approaches, a common foundation is needed. In this paper we present a simple module calculus consisting of a small set of operators over environments and modules. Using these operators, we are then able to specify a set of module combinators that capture the semantics of Java packages, C# namespaces, Ruby modules, selector namespaces, gbeta classes, classboxes, MZScheme units, and MixJuice modules. We develop a simple taxonomy of module systems, and show how particular combinations of module operators help us to draw sharp distinctions between classes of module systems that share similar characteristics.