A double-sheet detector can either be mechanical or electric, and is attached to a caliper roller placed between the forwarding rollers. When paper beyond a certain preset thickness passes beneath the roller, the detector shuts off the feeding unit and/or the printing unit. Electric detectors have a set of electrical contacts that, when paper beyond a certain thickness passes beneath the caliper roller, are brought into contact, tripping the shutoff of the feeder. Needless to say, the sheet detector needs to be set precisely to the thickness of the paper being printed. On a stream feeder, in which the sheets overlap while feeding, it is necessary to set the double-sheet detector to trip when three sheets pass beneath it. When printing larger-size paper with a stream feeder, however, is it not uncommon for there to be points when three sheets overlap. Consequently, the double-sheet detector needs to be set to trip when four sheets pass beneath it. (On presses equipped with stream feeders, double-sheet detectors are commonly referred to as extra-sheet detectors.)
A double-sheet detector is also used on some folding machines as a means of detecting when more than one sheet is being fed into the mechanism.