Rigging screw

A tensioning device of shrouds and the like. It has two eye-bolts threaded into opposite ends of a central body. One bolt is threaded left-handed and the other right-handed, so that when the body is turned the ends move simultaneously either in or out. (That’s to say they move in opposite directions.) Rigging screws are the norm on modern boats for tensioning the rigging. They are commonly called bottle screws, though small ones may be called turnbuckles.