Pulley, a device consisting of a wheel, usually with a grooved rim, that is used with a rope or chain. A pulley is a simple machine. A pulley block, or block, is a case that encloses one or more pulleys (often also called sheaves) and has a hook or eye by which it can be attached. The rope or chain used with a pulley is often called the tackle.
A fixed pulley only changes the direction of the force applied; it has a theoretical mechanical advantage of 1. (The theoretical mechanical advantage is the ratio of the force delivered by the machine to the force put into it, disregarding friction.) When one end of the rope is attached to a support and a movable pulley is used, the theoretical mechanical advantage (assuming that the pulley itself does not weigh anything) is 2. A block and tackle is a combination of fixed and movable pulleys with a single rope passing over the sheaves. It has a theoretical mechanical advantage equal to the number of lengths of rope that support the movable pulleys.