For handheld computers, there are basically two UI design patterns. The first one, the keyboard UI, has been almost invented and immediately perfected by Psion in their Series 3xx palmtop computer line. It really would take far too long to explain how the Psion UI works (although, as a longtime Psion afficionado, I’m sorely tempted!) but let’s suffice by saying that the keyboard-based user interface of those computers was simple, intuitive (as far as any computer metaphore can be intuitive),easy to learn and as frugal as possible. That last property means that the interface was designed in such a way that any user action could be performed with as little input as possible (I’d almost dare to claim that Psion managed to find the path with the least possible inputs for everything — I sure like to see proof of the opposite).