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Big hands mean big buttons

Fitt's Law (and more generally, Steering Law) has strongly influenced human-computer interface design. It's the reason why high-value click targets are placed at the edges of the screen: they are easier to hit because you can slam the pointer against the edges. It also explains why deep hierarchical menus suck: the longer the path you have to steer the pointer through, the more likely it is you'll make a mistake.

Computers went through a long period when displays became larger and denser while pointing technology become more accurate. We could present more information at once; toolbars grew and sprouted palettes and ribbons and submenus. Programmers came to assume this trend would continue.

Then cell phones mutated into real pocket computers. So far so good; we know how to deal with small screens. It's going to be a pain trying to fit all the information people now expect into a smaller area, but it's possible. But these small screens are also touchable. The union of what you see and what you touch breaks a lot of assumptions about how we design interfaces.

Steering Law doesn't apply when the user can tap one point and then another without having to steer through a path in between. The importance of large click targets goes way up because we're using our big fat fingers instead of a one-pixel pointer. The pointer model doesn't take into account how the finger and hand obscure other parts of the screen. The importance of edge and corner click targets goes down because a virtual edge cannot stop your real finger.

A new interaction model might also need to take into account handedness and fatigue. A while ago I wrote a Morse code "keyboard" for my mobile phone to see how it compared to awkward QWERTY keyboards [1]. It had a dit button on the left and a dah button on the right. Within minutes of using it I found an annoying bias towards dits in the Morse alphabet. On a telegraph the dit is 2/3 faster than the dah so naturally the code's designer used them more. The surprising part was how uncoordinated my left hand was and how quickly it got tired.

And then you have multitouch, which knocks "mouse gestures" into a cocked hat, provided we can figure out the right way to do it.

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