Apple's U.S. Patent No. 8,462,133 for "Clickable and tactile buttons for a touch surface" describes a method in which removable, deformable buttons are mounted to a touch-sensitive surface in order to give users a physical form of feedback.
In one embodiment, the "attachment structure," as Apple calls it, is fitted to a touchscreen like that of an iPhone or iPad, using suction to create a tactile button overlaid atop the display. This button can be made to deform when pressure is applied, thus giving the user positive feedback that the actuator was depressed.
Key to the patent is the buckling motion provided by the shape of the button. Some embodiments call for a dome shape which, when depressed, will buckle and when released will "unbuckle," or snap back to its original form. The action provides a tactile "click" sensation for a user, something touchscreens lack.
Because the button is made of rubber, or similar transparent material, the user can see through to the display beneath. In other implementations, the material is opaque and blocks the portion of screen it covers.
As for touch registering, the patent notes that a puck made out of metal or doped rubber may be disposed within the button dome, which will bridge capacitive elements in a touchscreen display when it comes in contact with the screen's surface. The size of the puck can be varied depending on usage.
Illustration of dielectric puck.
Additionally, more than one button can be attached to a screen's surface. For example, a joystick arrangement is described in the patent language which has a plastic shaft and suitable contact points setup as directional arrows.
Finally, single or multiple buttons can be used as finger rests which protect from inadvertent touches, allowing only assertive actions to be registered.
Apple's button apparatus patent was first filed for in 2008 and credits Stephen Brian Lynch, John Benjamin Filson and Fletcher R. Rothkopf as its inventors.