A Tuning Post (túnɪŋ póst) is also known as a “tuning key,” or “machine head,” which is the part of the guitar that attaches the strings to the head of the guitar. A tuning post is used to increase or decrease the tension of a guitar string. The tuning post consists of a cylinder mounted on a pinion gear, linked with a knob and joined together with a “worm gear.” The guitar string goes through a hole in the cylinder, so turning the knob tightens or loosens the string. The gears lock the knob in place to keep it from spinning freely after tightening. Changing a string’s tension affects its pitch, so players can adjust the string until it is properly in tune.

Six-string guitars feature six tuning posts, usually three on each side of the headstock. This is called the “3+3 Layout” and is the most common.