Abstract: A slide bar for guiding the carriage of a measuring or tracing device along a side of a levelling plate. The slide bar includes a guide rail which is divided into a first strip and a second strip having respective guide faces, the first and second strips being connected by a connector capable of acting as a biaxial joint, and the second strip being positionable adjustably with respect to the levelling plate by means of securing devices and distance pieces such that it may be brought exactly into parallelism with a reference line on the levelling plate.