• Make it easy to determine what actions are possible at any moment.
• Make things visible, including the conceptual model of the system, the
alternative actions, and the results of actions.
• Make it easy to evaluate the current state of the system.
• Follow natural mappings between intentions and the required actions; between
actions and the resulting effect; and between the information that is visible
and the interpretation of the system state. (Norman, 1988, p.188)