Principle of least astonishment

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In user interface design, programming language design, and ergonomics, the principle (or rule) of least astonishment (or surprise) states that, when two elements of an interface conflict or are ambiguous, the behaviour should be that which will least surprise the human user or programmer at the time the conflict arises, because the least surprising behavior will usually be the correct one.

For example, a user interface may have the behaviour that pressing Control-Q causes the program to quit. The same user interface may have a facility for recording macros, or sequence of keystrokes to be played back later, intended to be able to control all aspects of the program. The user may want to record a keystroke sequence with Control-Q as part (most likely the last part) of the macro. The principle of least surprise says that pressing Control-Q while recording a macro should not quit the program (which would surprise the user), but rather to record the keystroke.

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