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26 Chapter 1 What is interaction design?

interfaces are better conceptualized as perceived affordances, which are essentially
learned conventions. In conclusion, Norman argues that other design concepts--con-
ventions, feedback and cultural and logical constraints-are far more useful for help-
ing designers develop graphical user interfaces.

1.6.1 Heuristics and usability principles

When design principles are used in practice they are commonly referred to as
heuristics. This term emphasizes that something has to be done with them when
they are applied to a given problem. In particular, they need to be interpreted in
the design context, drawing on past experience of, for example, how to design feed-
back and what it means for something to be consistent.
Another form of guidance is usability principles. An example is "speak the user's
language." These are quite similar to design principles, except that they tend to be
more prescriptive. In addition, whereas design principles tend to be used mainly for
informing a design, usability principles are used mostly as the basis for evaluating
prototypes and existing systems. In particular, they provide the framework for heuris-
tic evaluation (see Chapter 13). They, too, are called heuristics when used as part of