A NUT WITH A C-SHAPED PROFILE, wherein the upper base (10), has a hole with at least one thread for the arrangement of the screwed device and wherein the lower base (11), which is aligned with the axis of the preceding hole, has another hole for receiving a nut tightening screw, said upper base being provided with incisions which define horizontal connecting portions (8) separated from the body of said upper base and longitudinal connecting portions (7) which form an extension of the former and which join together said upper base (10) and lower base (11), which lower base (11) has a retaining claw (1) designed to facilitate application of a tool for slackening the pressure which the nut applies to the panel to which it is fixed; which nut has a bottom rear flange (2) and has side stops on said upper base (10) which form an anti-rotation mechanism when a screw is tightened on the nut; said bottom rear flange (2) freeing from pressure said upper base (10) during assembly on the panel; and said longitudinal connecting portions (7) are overdimensioned.