Mode error

In user interface design, a mode is a distinct setting within a computer program or any physical machine interface, in which the same user input will produce perceived results different to those that it would in other settings. The best-known modal interface components are probably the Caps lock and Insert keys on the standard computer keyboard, both of which put the user's typing into a different mode after being pressed, then return it to the regular mode after being re-pressed.

An interface that uses no modes is known as a modeless interface. Modeless interfaces avoid mode errors by making it impossible for the user to commit them.

A precise definition is given by Jef Raskin in his book The Humane Interface:

"An human-machine interface is modal with respect to a given gesture when (1) the current state of the interface is not the user's of attention and (2) the interface will execute one among several different responses to the gesture, depending on the system's current state." (Page 42).

In Raskin's sense and according to his definition, an interface is not modal as long as the user is fully aware of its current state. Raskin refers to this as "locus of attention" (from the Latin word locus meaning "place" or "location"). Typically a user is aware of a system state if the state change was purposefully initiated by the user, or if the system gives some strong signals to notify the user of the state change in the place where interaction occurs. If the user's locus of attention changes to a different area, the state of the interface may then represent a mode since the user is no longer aware of it.

Several examples of well-known software have been described as modal and/or using interface modes:

Modes are often frowned upon in interface design because they are likely to produce mode errors when the user forgets what state the interface is in, performs an action that is appropriate to a different mode, and gets an unexpected and undesired response. A mode error can be quite startling and disorienting as the user copes with the sudden violation of his or her user expectations.

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