Understanding sets of classes, or packages, is an important activity in the development and reengineering of large object-oriented systems. Packages represent the coarse-grained structure of an application. They are artefacts to deploy and structure software, and therefore more than a simple generalization of classes. The relationships between packages and their contained classes are key in the decomposition of an application and its (re)-modularisation. However, it is difficult to quickly grasp the structure of a package and to understand how a package interacts with the rest of the system. We tackle this problem using butterfly visualizations, i.e., dedicated radar charts built from simple package metrics based on a language-independent meta-model. We illustrate our approach on two applications and show how we can retrieve the relevant characteristics of packages.