Abstract: The elements of the frame, such as A, B, C, and D are held in place by the four angles (1) disposed at the corners of the frame and by the cramps (5, 6, 7, 8) mounted on each angle. On the other hand, each angle comprises a semi-circular flat sheave (1e) surrounding its right-angle apex. The edge of this sheave is provided with a groove (4) in which the cable (3) passes. The tightener (2) comprises a clamp (13) tightened by a screw (16) permitting the two strands of the cable (3) to be clamped between its body and its gripping part. The nut (23) bearing against the end side of the frame (10) acts upon the threaded rod (22) and exerts a pull on the clamp (13) through which the cable is tightened and the elements of the frame are squeezed tight against one another.