The invention provides a screw head having a hexalobular slot (2) in the upper surface of the screw head, an upper peripheral wall (9) of hexalobular planform extending down into the screw head, the upper peripheral wall (9) being parallel or near parallel to the axis of the screw, a sloping transition surface (15) extending downwardly and inwardly with respect to the lower edge of the upper peripheral wall (9), and a tapered recess (11) extending downwardly from the lower edge of the sloping transition surface (15), in which the dimensions of the screw head are such that when the screw head is engaged by a tool (6) having upper (16) and lower engagement (17) sections, the lower engagement section (17) is tapered, and there is a ‘stick fit’ between the surface of the tapered recess (11) and the external surface of the lower tapered engagement section (17), and there is no engagement with the sloping transition surface (15).