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1.6 More on usability: design and usability principles 21

do next in their tasks. Design principles are derived from a mix of theory-based
knowledge, experience, and common sense. They tend to be written in a prescrip-
tive manner, suggesting to designers what to provide and what to avoid at the inter-
face-if you like, the do's and don'ts of interaction design. More specifically, they
are intended to help designers explain and improve the design (Thimbleby, 1990).
However, they are not intended to specify how to design an actual interface (e.g.,
telling the designer how to design a particular icon or how to structure a web por-
tal) but act more like a set of reminders to designers, ensuring that they have pro-
vided certain things at the interface.
A number of design principles have been promoted. The best known are con-
cerned with how to determine what users should see and do when carrying out
their tasks using an interactive product. Here we briefly describe the most common
ones: visibility, feedback, constraints, mapping, consistency, and affordances. Each
of these has been written about extensively by Don Norman (1988) in his bestseller
The Design of Everyday Things.

Visibility The importance of visibility is exemplified by our two contrasting exam-
ples at the beginning of the chapter. The voice-mail system made the presence and
number of waiting messages invisible, while the answer machine made both aspects
highly visible. The more visible functions are, the more likely users will be able to
know what to do next. In contrast, when functions are "out of sight," it makes them
more difficult to find and know how to use. Norman (1988) describes the controls
of a car to emphasize this point. The controls for different operations are clearly
visible (e.g., indicators, headlights, horn, hazard warning lights), indicating what
can be done. The relationship between the way the controls have been positioned
in the car and what they do makes it easy for the driver to find the appropriate con-
trol for the task at hand.

Feedback Related to the concept of visibility is feedback. This is best illustrated
by an analogy to what everyday life would be like without it. Imagine trying to play
a guitar, slice bread using a knife, or write using a pen if none of the actions pro-
duced any effect for several seconds. There would be an unbearable delay before
the music was produced, the bread was cut, or the words appeared on the paper,
making it almost impossible for the person to continue with the next strum, saw, or
stroke.
Feedback is about sending back information about what action has been done
and what has been accomplished, allowing the person to continue with the activity.
Various kinds of feedback are available for interaction design-audio, tactile, ver-
bal, visual, and combinations of these. Deciding which combinations are appropri-
ate for different kinds of activities and interactivities is central. Using feedback in
the right way can also provide the necessary visibility for user interaction.

Constraints The design concept of constraining refers to determining ways of re-
stricting the kind of user interaction that can take place at a given moment. There
are various ways this can be achieved. A common design practice in graphical user
interfaces is to deactivate certain menu options by shading them, thereby restrict-