A pulley, also called a sheave, is a mechanism composed of a wheel or shaft that may have a groove between 2 flanges around its circumference. A belt, or chain usually runs over the wheel and inside the groove, if present. Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion. It is 1 of the 6 simple machines. Two or more pulleys together are called a block and tackle.