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 from those that it would in other settings. Modal interface components include the Caps lock and Insert keys on the standard computer keyboard, both of which typically 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, in which the user performs an action appropriate to one mode while in another mode, by making it impossible for the user to commit them. Definition In his book The Humane Interface, Jef Raskin defines modality as follows: "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 locus 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. Larry Tesler defined modes as "a state of the user interface that lasts for a period of time, is not associated with any particular object, and has no role other than to place an interpretation on operator input." Examples Modal Several examples of software have been described as modal or using interface modes: Text editors – typically are in insert mode by default but can be toggled in and out of overtype mode by pressing the Insert key. Bravo (editor) – the first WYSIWYG modal editor made for Xerox Alto computers at Xerox PARC by Butler Lampson and Charles Simonyi vi – has one mode for