Users act according to a mental model: their current understanding of what the system contains, how it behaves and what actions will cause.

Designers provide a conceptual model through the interface. Names, structure, controls and feedback suggest what objects exist and how those objects relate.

When the visible model matches the user's task, behavior becomes easier to predict. A file interface that presents folders and documents, for example, gives users stable objects they can move, rename and inspect.

Problems arise when implementation details leak into the conceptual model without helping the task. Requiring users to understand internal database identifiers or hidden state transitions increases the knowledge needed to operate the system.

Consistency helps because knowledge learned in one place transfers to another, but consistency with a misleading model merely makes the same misunderstanding repeatable.

Good conceptual models make the important system state and relationships understandable without requiring users to reconstruct the implementation.