A device often found on string instruments (the violin, guitar, and bass included] that connects to a machine head. Turning the peg allows the player to raise or lower the pitch of a certain string. The peg is usually flat and thick enough that it can be easily turned using only the player's thumb and index finger.

On a guitar, the wide part of the peg is attatched to a stem which is a small worm gear. This worm gear is threaded into the grooves of a small circular gear with many teeth. The small gear is attatched to the cylinder around which the string is wound. Turning the peg results in string tension either increasing or decreasing, increasing and decreasing the pitch, respectively.

The peg to worm gear to circular gear is vital in that it both reduces the amount of force it takes to change the pitch of the string, as well as the amount that the string is tightened per each rotation of the peg. This allows the string to be finely tuned and prevents the musician from easily stretching the strings way out of tune or completely breaking them.

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