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22 Chapter 1 What is interaction design?

Figure 1.8 A menu illustrating restricted availability of options as an example of logical
constraining. Shaded areas indicate deactivated options.

ing the user to only actions permissible at that stage of the activity (see Figure 1.8).
One of the advantages of this form of constraining is it prevents the user from se-
lecting incorrect options and thereby reduces the chance of making a mistake. The
use of different kinds of graphical representations can also constrain a person's in-
terpretation of a problem or information space. For example, flow chart diagrams
show which objects are related to which, thereby constraining the way the informa-
tion can be perceived.
Norman (1999) classifies constraints into three categories: physical, logical, and
cultural. Physical constraints refer to the way physical objects restrict the move-
ment of things. For example, the way an external disk can be placed into a disk
drive is physically constrained by its shape and size, so that it can be inserted in
only one way. Likewise, keys on a pad can usually be pressed in only one way.
Logical constraints rely on people's understanding of the way the world works
(cf. the marbles answering machine design). They rely on people's common-sense
reasoning about actions and their consequences. Picking up a physical marble and
placing it in another location on the phone would be expected by most people to