The sixteenth law is Consistency: The interface will be consistent.
When an interface is inconsistent, users cannot predict what will happen next or what their next action should be. People need consistency in order to perceive the underlying conceptual model, and to adjust their mental model to better fit the conceptual model. If the interface is consistent, this adjustment is more easily made.
People need consistency.
Technology Considerations and Implications
Consistency includes not only terminology, but the feel, behaviors, and actions of the system itself, and the actions required of the user.
For example, if the user has learned that “Go Forward” is a valid command, the mental model would suggest that “Go Back” is a corollary command. If, instead, you use “Previous,” you are being inconsistent and making it harder for the user to create a mental model that allows them to predict what to do next.