Double-sheave block with a rope becket and hook, used aboard ship. The becket is the part that goes around the block and allows it to be attached to the object on which force is being exerted. When a becket has a hook spliced into it, the block and tackle can be moved or shifted. If it needs to be semi-permanently fastened, the hook is hooked into the sail or other object and a mousing or lashing is made from the point of the hook back to the body.
Blocks can be made by hand but making blocks was the first woodworking project that was highly mechanized with special block-making machinery invented to provide the thousands of blocks needed for the Royal Navy during the Napoleonic Wars.