Textbook of Usability

Norman's design principles, articulated in Don Norman's The Design of Everyday Things (1988, revised 2013), explain the psychological mechanisms behind good and bad design. Unlike Nielsen's evaluative heuristics or Shneiderman's design rules, Norman's principles focus on why designs succeed or fail.

The core principles:

• Affordances: the relationship between an object's properties and the capabilities of the agent that determines how the object could be used. A flat plate affords pushing; a handle affords pulling.
• Signifiers: perceivable indicators of affordances — labels, icons, textures, visual cues. Signifiers tell users where to act and what to do.
• Mapping: the relationship between controls and their effects. Natural mapping exploits spatial correspondence (a row of switches matching a row of lights).
• Feedback: immediate, informative, perceptible response to every action
• Constraints: physical, logical, semantic, and cultural limits that prevent invalid actions
• Conceptual models: simplified explanations of how the system works, supporting prediction

Norman also distinguishes the designer's model, the system image, and the user's model — and observes that the designer communicates with the user only through the system image. The gulf between these three is where usability problems live.

Norman's framework is particularly powerful for analysing physical products as well as software, because the principles apply universally: a door handle follows the same rules as a button in a GUI.