A tuning peg is used to hold a string in the pegbox of a stringed instrument. It may be made of ebony, rosewood, boxwood or other material. Some tuning pegs are ornamented with shell, metal, or plastic inlays, beads (pips) or rings.
By turning the peg, one winds/unwinds the string, making it tighter/looser. This affects the pitch produced when the string is sounded, thereby tuning it. When tuning, the player adjusts the tension of the peg by slightly pressing in or pulling out along the peg's axis while turning the peg. Peg dope, applied to the shaft, eases the tuning process.
A properly working peg will turn easily, and hold reliably, that is, it will neither stick nor slip. Modern pegs for violin family instruments have conical shafts, turned to a 30:1 taper, changing in diameter by 1 mm over a distance of 30 mm. A peg may become worn so that it is no longer evenly conical, showing slight depressions on the bearing surface where it contacts the cheeks of the pegbox. This sort of wear makes tension adjustment difficult to impossible, and the peg may slip. When this happens, or when the pegs have sunk in too far, new pegs are in order, perhaps along with bushings for their holes.