A few years ago Microsoft’s user interface researchers suggested that there may be a better use for the back of a device besides covering the battery, and suggested the back surface of a phone may be made touch sensitive and used to control whatever is going on on the screen.
A simple version of this, for example, would be how the fingerprint sensor can be used on Samsung devices to pull down the notification shade or for other purposes.
Of course, Microsoft no longer has a phone business to implement these ideas, but Christian Klein and other Microsoft researchers realized that in some configurations, a folded Surface Phone would automatically offer a touch-sensitive surface on the back of the device and that not using it would be a wasted potential, leading to their newly published patent for a “Gesture Language for a Device with Multiple Touch Surfaces”.
The patent notes:
A gesture language for a device with multiple touch surfaces is described. Generally, a series of new touch input models is described that includes touch input interactions on two disjoint touch-sensitive surfaces. For example, a mobile device can include a primary display on a “front” side of the device, and a secondary display or touch-sensitive surface on the “back” side of the device, such as a surface that is opposite the primary display. Accordingly, the gesture language can include a series of “back touch” interactions with the touch-sensitive surface on the backside of the device. Example interactions include direct and indirect touch input on the back side, as well as simultaneous touch input on both sides of the device.
A very rich vocabulary of gestures are of course possible, but at its simplest we could have scroll and swipe gestures which could be executed easily with one hand while still having a firm grip on your phone, making triaging email or reading long web pages easier.
The patent does, of course, have other applications besides Project Andromeda aka the Surface Phone. You can read full details in the patent application here.