Abstract: An arrangement for fixing a clamp to a C-shaped rail comprises a fixing plate urged against inner arms of the rail by a compression spring, the plate having a width which fits between inner arms so that it can be inserted from above, and a length, which fits between the arms of the C-shape, diagonally opposite portions of the ends of the plate being shaped to permit it to rotate between the arms of the rail. The spring is located between the head of a spigot fixed to the fixing plate and a supporting plate. The ends of inner arms are arranged to be located in grooves in the fixing plate.