In some cases we want to be able to enter a value in a number of different ways. Consider a graphic which needs to represent the length of some object. The length could be entered in metric or imperial units, and it may be useful to always see the dimension output in both forms.

In the diagram shown, if a new metric value is entered, the imperial measure is automatically updated to match it. The metric value is an input and the imperial measure is an output. We are also allowing the user to do the opposite; to input the imperial value and the metric value will be automatically updated.