The user has to know how to control the character while in the game. The level of instruction depends of the number of actions a character possesses.

For example, in "Pong" the user may move the bat up or down, so the number of instructions needed are extremely limited.

However, in "Sim City 3000", the number of possible user actions are in the hundreds. When activated, each of the icons on the screen allow a combination of actions, so without detailed instructions the game would be very difficult to play.
	Click to enlarge

Controls must be listed in a logical sequence, and available at all times while the user is controlling his or her character. An interface fails when a user wishes to take an action, but does not know how to and cannot discover how to.