Abstract: A system for interlocking a plurality of modules, such as medical instrument modules (e.g., infusion pumps), in side-by-side relationship such that not more than one module can be mounted on each side of a center module. The modules are interchangeable in that any module of the system can be a center module or a side module. In the preferred embodiment, latching arms on a module are automatically moved as the module is mounted on a pole stand to allow other "side" modules to be mounted on that "center" module. The latching arms of the "side" modules prevent additional modules from being mounted on the side modules. Other embodiments are described as well.