Design your software so that users can directly manipulate software representations of information. Whether dragging an object to relocate it or navigating to a location in a document, users should see how the actions they take affect the objects on the screen. Visibility of information and choices also reduce the user's mental workload. Users can recognize a command easier than they can recall its syntax.
Familiar metaphors provide a direct and intuitive interface to user tasks. By allowing users to transfer their knowledge and experience, metaphors make it easier to predict and learn the behaviors of software-based representations.
When using metaphors, you need not limit a computer-based implementation to its "real world" counterpart. For example, unlike its paper-based counterpart, a folder on the Windows desktop can be used to organize a variety of objects such as printers, calculators, and other folders. Similarly, a Windows folder can be more easily resorted. The purpose of using metaphor in the interface is to provide a cognitive bridge; the metaphor is not an end in itself.
Metaphors support user recognition rather than recollection. Users remember a meaning associated with a familiar object more easily than they remember the name of a particular command.
For information about applying the principle of directness and metaphor, see Chapter 5, "General Interaction Techniques," and Chapter 13, "Visual Design." These chapters cover, respectively, the use of directness in the interface (including drag and drop) and the use of metaphors when designing icons or other graphical elements.