We present a new set of metrics for analyzing the interaction between the modules of a large software system. We believe that these metrics will be important to any automatic or semi-automatic code modularization algorithm. The metrics are based on the rationale that code partitioning should be based on the principle of similarity of service provided by the different functions encapsulated in a module. Although module interaction metrics are necessary for code modularization, in practice they must be accompanied by metrics that measure other important attributes of how the code is partitioned into modules. These other metrics, dealing with code properties such as the approximate uniformity of module sizes, conformance to any size constraints on the modules, etc., are also included in the work presented here., To give the reader some insight into the workings of our metrics, this paper also includes some results obtained by applying the metrics to the body of code that constitutes the open-source Apache HTTP server. We apply our metrics to this code as packaged by the developers of the software and to the other partially and fully randomized versions of the code.