A machine head is a geared apparatus for tuning stringed musical instruments by adjusting string tension. Machine heads are used on mandolins, guitars, double basses and others, and are usually located on the instrument's headstock. Other names for guitar tuners include pegs, gears, machines, cranks, knobs, tensioners and tighteners.
Open worm type machine head on a ukulele
Machine heads on a double bass
The machine heads on a classical guitar. Note the exposed gears and the decorations.
Martin EB18 bass guitar headstock, showing Martin open-type machine heads.
A headstock or peghead is part of a guitar or similar stringed instruments such as a lute, mandolin, banjo, ukulele and others of the lute lineage. The main function of a headstock is to house the tuning pegs or other mechanism that holds the strings at the "head" of the instrument; it corresponds to a pegbox in the violin family. At the "tail" of the instrument the strings are usually held by a tailpiece or bridge. Machine heads on the headstock are commonly used to tune the instrument by adjusting the tension of strings and, consequently, the pitch of sound they produce.
Classical guitar headstock
Bass guitar headstock
Headstock from an ARTCORE series guitar by Ibanez
Ibanez JEM 555 BK headstock