Abstract: A coin validator is operable in a set up mode prior to normal operation, in which initial window data (W) stored in its memory, is compared with data from a known true coin, and the initial window is progressively dragged and shrunk depending on the outcome of the comparison, to produce an operating window (W′), narrower than the initial window, which can be used during normal operation of the validator, for comparison with coin data (x) from coins under test, in order to determine coin acceptability. The initial window (W) can be the same for all validators of the same design, and the dragging and shrinking configures the operating window (W′) to the validators individually, to take account of manufacturing tolerances.