The notion of functional or modular dependency is fun- damental to understand the architecture and inner work- ings of any software system. In this paper, we propose to extend that notion to consider dependencies at a larger scale, between software applications (usually programs or libraries themselves). These dependencies, which we call inter-dependencies are of exceptional importance in free an open source software (FOSS), where it is common to build new applications by taking advantage of a rich and com- plex environment of programs and libraries whose function- ality is available. To explore this concept, a methodology and visualization for studying inter-dependencies of a com- plex software system is presented and applied to one of the largest distributions of FOSS: Debian GNU/Linux.