A clamping system with a clamping nut and a wrench for tightening the clamping nut, wherein the clamping nut includes at least one recess on either the outside edge or the inside edge of the end face of the clamping nut to receive projections of the wrench. The edge opposite the edge with at least one recess is closed. The at least one recess is provided with at least one undercut, which the projections of the wrench can grip from underneath.