The prime mover runs a set of pulleys to the transmission, often a double-reduction gearbox, which drives a pair of cranks, generally with counterweights installed on them to offset the weight of the heavy rod assembly. The cranks raise and lower one end of an I-beam which is free to move on an A-frame. On the other end of the beam is a curved metal box called a horse head or donkey head, so named due to its appearance. A cable made of steel—occasionally, fibreglass—, called a bridle, connects the horse head to the polished rod, a piston that passes through the stuffing box.