Modules vary only in width to fit each cage's module bays.
Mouse cages are narrower than rat cages. Most types of modules are available in both widths. Mouse modules are, for the most part, scaled-down versions of the rat modules. Most of the devices on the modules are confined to the space projected rearward by the face dimensions, thus permitting most modules to occupy any position and be adjacent to most other modules. No module, including a retreat box or runway, will interfere with any other module in any hub owing to the radial array of the module bays.