An expandable keyboard for a pocket-type calculator is made in modular form to electrically interconnect to form a keyboard much larger than that of a familiar pocket calculator. The keyboard portions are hinged together normally forming a stack, which occupies along with the display and the electronics,...http://www.google.es/patents/US3940758?utm_source=gb-gplus-sharePatente US3940758 - Expandable keyboard for electronic pocket calculators and the like
Expandable keyboard for electronic pocket calculators and the like
