Abstract: Supplementary jaw means is described for fitting to a clamping workbench transversely to the main jaws thereof at a position spaced from stop means at the other end of the main jaws, whereby operation of the drive means which moves the main jaws towards or away from each other will increase the distance between the supplementary jaw means and the stop means when operated in one sense, and will decrease the distance therebetween when operated in the other sense. A clamping workbench may be fitted with one supplementary clamping jaw means and a fixed stop for clamping a workpiece therebetween above the main clamping jaws.