An interface must communicate both what actions are possible and how controls relate to their effects.

An affordance is an action the object permits; a signifier is a perceivable cue that communicates where or how to act. A button may afford pressing while its shape, label and placement signify that action.

A mapping connects a control to its result. Controls are easier to learn when the mapping is spatially or conceptually natural—for example, an arrow pointing right to move forward through a horizontal sequence.

Constraints reduce invalid actions. A control can be disabled when its action is unavailable, an input can restrict values to the allowed domain, and physical arrangement can make incorrect choices harder.

These mechanisms reduce the amount of hidden knowledge users must remember. An interface should not rely on convention alone when the system can make the available action and its consequence visible.