Used in the control of a process, the controller output is proportional to the error or a change in a process variable. The proportional band is the fractional or percentage change in input that is required to produce a 100 per cent change in controller output. The proportional band is therefore a value of 100 divided by the gain of the process. For example, a 100 per cent controller output may be a signal that fully opens a valve, while a 0 per cent controller output fully closes it.