<p>A thread-forming screw (10) has first (20), second (22) and third (24) threads parallel to each other to extend like a helix over the length of a cylindrical shaft (16). The threads have different external diameters (d1-d3). The external diameter (d1) of the first thread is larger than the external diameter of the second thread that follows the first thread in a setting direction. The external diameter of the second thread is larger than the external diameter of the third thread that follows the second thread in a setting direction.</p>
