Abstract: On a printed circuit board or module card (20) of a standard size for a particular type of system, one or more different, smaller circuit cards or sub-modules (50) can be selectively mounted on the module card so as to utilize a maximum of its area. The module card has openings or holes (30) therein so as to provide component clearance and thus prevent the thickness of the module card from reducing the maximum permissible height of the components that can be mounted on the sub-modules. In order to minimize the amount of module card area devoted to interconnecting to the sub-modules, the sub-modules plug into the module card as the sub-module is moved in a direction perpendicular to the plane of the module card rather than in a direction parallel to the plane of the module card. Circuit wiring and switching (FIG.