Visibility. Controls should be clearly visible, so users can see the controls that are available to them. Visual feedback should also be clearly visible, so users can understand what action has been done and what effect has been achieved, and hence what needs to be done next. For example, car indicators are often activated by lifting up or pressing down a stalk attached to the steering wheel. In some cars this stalk is partially obscured by the steering wheel, which suggests the visibility of this control could be improved. However, this is usually acceptable to drivers because most car dashboards are designed in this way, so drivers can apply their knowledge of other cars in order to overcome this problem- this is an example of consistency. In my car there are two warning lights which indicate whether the right or the left indicators are flashing. These are clearly visible to the driver, as they are at the centre of the dashboard, in frontofthe driver'sseat.

In complex interactive products it is often difficult to achieve good visibility for all of the controls and visual feedback, because there are too many controls and too much information. In such circumstances it is necessary to prioritise based upon user needs, and give the most important the best visibility. For example, in my car, the low-oil warning light is clearly visible, but the lever to open the petrol cap is hidden away on the floor beside the driver'sseat.

Feedback. This is the information sent back to the user to confirm what action has been done and what result has been accomplished. Feedback can be visual (e.g. text on a display, flashing alert messages), auditory (e.g. beeps, bells or other noises such as key clicks as you press keys on a keypad or keyboard) or tactile (e.g. a button can be felt to move as it is pressed). The better the feedback, the easier it is to perceive what is happening and hence interact with the device.

Constraints. Constraints are ways of restricting the kind of interaction that can take place at a given moment. For example, when menu options are greyed out they cannot be selected or the steering column of a car restricts the movement of the steering wheel so it can only be rotated.

Effective constraints can help guide users through the interaction, helping them to avoid errors. Consistency. This is where a user interface is designed to follow certain rules, such as always selecting objects by clicking the left mouse button or always having the Help menu at the right-hand end of the menu bar.

There are different types of consistency. For example, internal consistency means the user interface is consistent within itself. For example, on the dashboard of a particular car, every warning light would be red, as opposed to some being red and others being yellow. In contrast, external consistency means the user interface is consistent with the external world. For example, on the dashboard of a particular car, the icon used to indicate that the fog lights are on is the same icon used throughout the whole range of cars produced by the same manufacturer. Consistency often helps make user interfaces easier to learn and user errors less likely.