A machine head (also referred to as a tuning machine, tuner, or gear head) is a geared apparatus for tuning stringed musical instruments by adjusting string tension. Machine heads are used on mandolins, guitars, double basses etc., and are usually located on the instrument's headstock. Non-geared tuning devices that are used on violins, violas, cellos, lutes, older Flamenco guitars, ukuleles etc., are known as friction pegs. Friction pegs hold the string in tune by way of friction caused by their tapered shape and by the string pull created by the tight string. Some other commonly used slang names for guitar tuners are pegs, gears, machines, cranks, knobs, tensioners and tighteners.

A machine head is part of the tuning mechanism of some stringed instruments.