Class 1 threads are a loose tolerance. Class 2 threads comprise 90% of stainless fittings and are normal commercial tolerance. Class 3 threads have a stricter tolerance and tighter fit such as socket cap and set screws. No definite relationship exists between tensile strength and tightness or looseness of fit. The symbol "A" added to threads, such as 2A, means external threads (screws), and "B" means internal (nuts).
A turnbuckle is a device for adjusting the tension or length of ropes, cables and tie rods, or other tensioning systems. It normally consists of two threaded fittings, one screwed into each end of a 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 eyelets to be screwed in or out simultaneously, without twisting the fittings or attached cables.