Abstract: In order to ensure correct registration of printing, a printing device is provided with a transducer arrangement which scans the printing platen to detect one of the edges of the stationary or, in the case of continuous feed stationery, pre-printed markings on the stationery. The scanning may take place in the vertical and/or horizontal directions according to the direction in which precise registration is desired. The transducer arrangement is provided with an associated light source and to avoid errors arising, e.g. from variations in ambient lighting level, the transducer arrangement comprises two sensors which are spaced apart in the direction of scanning and an associated circuit forms a difference signal from the outputs of the sensors, which is discriminated with respect to a predetermined threshold level to produce a signal representative of the edge or marking.