Abstract: A signature perforating knife that is useable to perforate multi-layer signatures is formed as a compound knife using either several knife blades or a combination of knife blades and spacer blades. Each compound perforating knife has a uniform base thickness. The specific characteristics of the compound perforating knife can be adapted in accordance with the weight of the paper web being printed and the number of layers of the multi-layer signature which will be perforated and then folded in a folding apparatus of a web-fed rotary printing press.