Abstract: A spring-biassed nut includes a generally rectangular nut body which has a spring device fitted on one main surface thereof. The nut can be fitted into a slotted hollow rail by inserting the nut through the rail slot in the direction of its plane. The spring tightens the nut against the rail legs defining the rail slot, so that the nut will be held in a chosen position along the rail by friction. A pivot point around which the long-edge flange of the nut pivots when the nut is first inserted into the rail is defined in the interior of the rail, preferably in the inner corner region between the rail side-wall and the flange. The length of the nut as measured between the edges of the long-edge flanges is essentially equal to or slightly smaller than the distance between the pivot point and the edge of the opposing rail flange.