We make two observations about communications middleware: first, most middleware are similar, the differences are in their interfaces and optimizations; second, neither a fixed set of abstractions nor a fixed implementation of a set of abstractions is likely to be sufficient and well-performing for all applications. Based on these observations, we present Quarterware, a customizable middleware architecture. It abstracts basic middleware functionality, and admits application specific specializations and extensions. We demonstrate its flexibility by deriving implementations for core facilities of CORBA, RMI, and MPI. The performance results show that the derived implementations equal or exceed the performance of corresponding native versions. These results suggest that customizing middleware on a per-application basis is an effective approach for building robust, high-performance applications.