Mary Rose Cook's notebook

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The Humane Interface, Jef Raskin

A human-machine interface is modal with respect to a given gesture when (1) the current state of the interface is not the user’s locus of attention and (2) the interface will execute one among several different possible responses to the gesture, depending on the system’s current state.

Both parts of the definition of a modal gesture are necessary to decide whether, for example, the gesture of pressing the Backspace key is modal. In most computer interfaces, assuming you are in the process of entering text, the Backspace command erases the most recently typed character. If that character was an e, the key erases an e. If that character was an x, the key erases an x. That is, sometimes Backspace is an e eraser, and sometimes it is an x eraser. Considering only the second part of the definition, the use of Backspace is modal because what it erases depends on the character most recently typed; context is part of the system state. However, when you realize that your locus of attention is the object that you are erasing, it is the first part of the definition that explains why the operation is not modal and why you do not make mode errors when you use the Backspace key.