Traditionally, a single machine head consists of a cylinder or capstan, mounted at the center of a pinion gear, a knob or "button" and a worm gear that links them. The capstan has a hole through the far end from the gear, and the string is made to go through that hole, and is wrapped around the capstan. To complete the string installation, the string is tightened by turning the capstan using the tuning knob. The worm gear ensures that the capstan cannot turn without a movement on the knob; it also allows precise tuning.