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.

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.