Abstract: The present invention teaches a female part that can locked in selected positions on a round rod. The female part includes a hollow passageway into which a locking screw can be screwed in a generally radial direction into engagement with the rod. When seen in a cross section, the passageway includes two flat surfaces which slope towards one another symmetrically in relation to a diametrical plane. The angle defined by the surfaces with the plane lies between 2.5' and 15.0'. The locking screw enters the passageway in the wider part between the surfaces.