• informs users about what is possible and what is not
• informs users about what will happen
• give users cues as to what action/input is correct
• indicates forseeable inconveniences for the user

Aim:    prevent users from performing actions that would lead to errors or otherwise unexpected results.
Form:  Advance information should be given through visual indicators rather than text.
 

Examples:

• Menu entries which currently cannot be selected should be visually distinguished from entries that can be activated.
• Objects in direct-interactive interfaces that currently cannot be accessed should be visually distinguished from objects that can be activated.
• The shape of the mouse symbol should indicate the action that will occur if the user clicks the mouse button.
• Submenus should be shown when the mouse moves over the superordinate items.
• Required formats of keyboard entries should be indicated.  
• Potential adverse side effects of actions should be pointed out if the user may not be aware of them, and the user's permission be requested.