Abstract: The modular structural base assembly is provided for constructing a custom key switch array which includes a foundation structural base module and a grouped keyswitch station arranged similarly to a typewriter key matrix. Accessory base modules of various keyswitch groupings are provided for attachment to the foundation module to permit addition of accessory keys at virtually any location and in any desired plurality. Accessory modules are coupled to the foundation module by a tongue-and-groove type connector which includes means for effectuating an interference fit to interlock adjacent modules. The tongue-and-groove couplers preferably have a dovetail shaped configuration.