Abstract: The position of a transverse crease line of a moving packing web is sensed in that light reflected from the web and emanating from a laser directed perpendicularly to the web is received by photo detectors from two directions in the direction of movement of the web and under an angle of for example 60.degree. in relation to the light beam directed to the web. The electrical measuring signal emanating from the photo detectors is compared in an electronic signal processor with a reference signal, which is stored in the processor and is representative of a reference crease line. A crease line is indicated when the value of a correlation signal, which constitutes a calculation of conformity between the measuring signal and the reference signal, reaches a maximum. In a corresponding way, the position of a longitudinal crease line can be sensed.