The invention relates to a jib-bracing system for a revolving tower crane, in the case of which the length of a jib (2) and the length of a bracing means (5) for the jib can each be changed. The bracing means comprises at least part of a cable (6) of which one end can be connected to the jib at at least two different fixed points (8, 9) arranged on the jib, wherein a deflecting element (7), over which the cable can be guided, is arranged on the jib, and wherein the one end of the cable can be fastened at at least one of the fixed points when the cable has been guided over the deflecting element.