A tactile or snap action membrane keyboard is presented wherein the tactile or snap action key elements are protrusions in a membrane sheet, each protrusion having a flat elliptical top surface or plateau with a plurality of inclined ramps from the base sheet to the plateau....http://www.google.se/patents/US4477700?utm_source=gb-gplus-sharePatent US4477700 - Tactile membrane keyboard with elliptical tactile key elements
