A
turnbuckle,
stretching screw or
bottlescrew is a device for adjusting the tension or length of
ropes,
cables,
tie rods, and other tensioning systems. It normally consists 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.