Object-oriented languages such as Java, Smalltalk, and C++ structure their programs using packages, allowing classes to be organized into named abstractions. Maintainers of large applications need to understand how packages are structured and how they relate to each other, but this task is very complex because packages often have multiple clients and different roles (class container, code ownership...). Cohesion and coupling are still among the most used metrics, because they help identify candidate packages for restructuring; however, they do not help maintainers understand the structure and interrelationships between packages. In this paper, we present the package fingerprint, a 2D visualization of the references made to and from a package. The proposed visualization offers a semantically rich, but compact and zoomable visualization centered on packages. We focus on two views (incoming and outgoing references) that help users understand how the package under analysis is used by the system and how it uses the system. We applied these views on three large case studies: JBoss, Azureus, and ArgoUML.