Parts of a Guitar Tuner – Knob, Worm, Gear, String Post

All 10 parts on the diagram

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From the back 4

Knob Turn it to tighten or loosen the string.
Shaft It joins the knob to the worm.
Worm A gear shaped like a screw. It turns the pinion gear.
Pinion gear The round gear fixed to the end of the string post.

From the side 6

String post The string winds around it as it turns.
String hole The end of the string goes through it.
Bushing A threaded sleeve through the headstock. The post turns inside it.
Washer A flat ring under the bushing that protects the wood.
Headstock The head of the guitar. The machine heads are mounted on it.
Housing It encloses the worm and the pinion gear.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two independent sources use it. (1) Wikipedia, Machine head, text only, https://en.wikipedia.org/wiki/Machine_head (a machine head, also called a tuning machine, tuner or gear head, has a cylinder or capstan in the middle of a pinion gear, a button or knob, and a worm gear linking them; the string goes through a hole in the capstan; the worm gear keeps the capstan from turning back; open gears on classical guitars, sealed enclosures with permanent lubrication on most steel-string guitars; mounted on the headstock). Three United States patents by different inventors, read at Google Patents: (2) US 8,093,475, Tuning device, https://patents.google.com/patent/US8093475B1/en (pinion gear, worm gear, knob or button, string post, string hole, a housing that encloses the gears, a bearing or bushing, head stock); (3) US 4,945,800, Stringed musical instrument machine head, https://patents.google.com/patent/US4945800A/en (housing, string post, worm and worm wheel, the worm's shaft out of the housing to the handle, a threaded tubular bushing that holds the machine head on the peghead, a washer under the bushing head to keep it from marring the peghead); (4) US 4,515,059, Geared tuning machine, https://patents.google.com/patent/US4515059A/en (knob, tuning shaft, worm gearing, gear wheel on the string-carrying post, housing, bushing, washer, peghead). No maker is named in the copy or drawn. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work

How it works

The parts of a guitar tuner, meaning the tuning machine on the headstock and not the electronic tuner. It is also called a machine head. One is drawn from the back with its housing open, then from the side.

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From the back, housing open

Numbers 1 to 4: the knob, the shaft, the worm and the pinion gear. Turning the knob turns the worm, and the worm turns the pinion gear.

From the side

Numbers 5 to 10: the string post with its string hole, the bushing and washer on the face of the headstock, the headstock itself and the housing behind it. The pinion gear is fixed to the post, so the post winds the string.

Tightening a string raises its pitch; loosening it lowers the pitch.

Questions and answers

What is the tuner on a guitar called?

A machine head. It is also called a tuning machine, a tuner, a tuning peg or a gear head. A six-string guitar has six, one for each string.

How does a guitar tuning machine work?

Turning the knob turns the worm. The worm turns the pinion gear, which is fixed to the string post, so the post winds the string tighter or looser. The worm also keeps the string from pulling the post back.

What is the difference between open and sealed tuners?

On open tuners the worm and the pinion gear can be seen. On sealed tuners they sit in a closed housing with permanent lubrication. Classical guitars usually have open gears; most steel-string guitars have sealed ones.

What does the bushing on a guitar tuner do?

It is the threaded collar on the face of the headstock. It screws in through the wood, holds the tuning machine in place, and gives the string post a sleeve to turn in. A washer under it protects the wood.