Turnbuckles are devices used to adjust the tension or length of ropes, cables, tie rods, and other tensioning systems. They typically consist of two threaded eye bolts, one screwed into each end of a small metal frame, one with a left-hand thread and the other with a right-hand thread. The tension can be adjusted by rotating the frame, which causes both eye bolts to be screwed in or out simultaneously, without twisting the eye bolts or attached cables.