In binding and finishing, any operation which punches tiny slits or holes in a sheet of paper or other substrate. Perforating is performed either on press or off press using perforating dies. Materials are perforated either to allow a portion to be easily removed (such as an order form or coupon), or to allow air to escape from folded signatures, which helps prevent wrinkling. The amount of paper between the perforations—called a tie—can be adjusted. Perforating may also be effected directly on an offset press by means of a perforating rule.