A physical keyboard key has three states, whereas a touch screen usually has only two. Due to this difference, the state corresponding to the touched state of a physical key is missing in a touch screen keyboard. This touched state is an important factor in the usability of a keyboard. In order to recover the role of a touched state in a touch screen, we propose the TapBoard, a touch screen software keyboard that regards tapping actions as keystrokes and other touches as the touched state. In a series of user studies, we validate the effectiveness of the TapBoard concept. First, we show that tapping to type is in fact compatible with the existing typing skill of most touch screen keyboard users. Second, users quickly adapt to the TapBoard and learn to rest their fingers in the touched state. Finally, we confirm by a controlled experiment that there is no difference in text-entry performance between the TapBoard and a traditional touch screen software keyboard. In addition to these experimental results, we demonstrate a few new interaction techniques that will be made possible by the TapBoard.
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