Abstract: In order to adapt a general-purpose calculator to specific tasks, the keyboard of that calculator is covered by a mask including a resilient membrane overlain by a rigid panel having cutouts registering with keys whose function is to be preserved. The cutouts receive extension keys glued or otherwise fastened to the membrane, depression of any extension key operating the underlying original key. The functions of the operative keys can be modified, in accordance with the contemplated use, by insertion of a stored-program cassette associated with the particular mask employed.