Consistency is one of the most basic usability principles and should be enforced throughout the system. The same information should be located in the same spot on all screens/dialog boxes and it should be formatted in the same way to facilitate recognition. Similarly, the same command or action should always have the same effect in equivalent situations. This encourages users to try out exploratory learning strategies since they can predict (with a good degree of confidence) how to operate new parts of the system. Finally, input should be consistent through the use of common syntax across a system.
Even though consistency becomes easier to achieve when following a user interface standard in the design, compliance to standards is not sufficient to ensure consistency since they allow the designers a fair amount of leeway.
It is also important to achieve consistency between the interface and the users' mental models. This provides consistency with the world as the user sees it (i.e. Simple and Natural Dialogue).
Good interfaces are not necessarily always consistent. It is possible to design "foolish consistency" where an interface makes the user carry out every operation in one way with no shortcuts for frequently used paths. Sometimes a trade-off must be made between consistency and a good product.