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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.
Friction pegs are most often used on violin family instruments (not on the double bass, which typically uses geared tuning machines.) They are also used on older instruments, such as the Bulgarian gadulka and the hurdy gurdy, as well as on flamenco guitars.
A properly working peg will turn easily and hold reliably, that is, it will neither stick nor slip. Modern pegs for violin and viola have conical shafts, turned to a 1:30 taper, changing in diameter by 1 mm over a distance of 30 mm. (Modern cello pegs have a slightly more aggressive 1:25 taper. 19th century and earlier pegs, for use with gut strings, typically had an even steeper taper of 1:20.)
The taper allows the peg to turn more easily when pulled out slightly, and to hold firmly when pushed in while being turned. Since the typical wear pattern on a peg shaft interferes with this action, pegs occasionally require refitting, a specialized job which amounts to reshaping both pegs and holes to a smooth circular conical taper.
Pegs for double bass and guitar family instruments are usually geared, and are called tuning machines or machine heads. Geared pegs for violin family instruments also exist, although they have not gained wide use, which has to do with the extensive and irreversible physical modification that must be made to the peg box in order to mount them, which is often viewed as ruining the aesthetics of the instrument, combined with a bad reputation they acquired due to poorly designed early models that were prone to failure, often with catastrophically damaging results. The most recently marketed pegs of this sort use planetary gears designed to fit inside a case shaped like a friction peg, and have seen some adoption as they require no more modification of the instrument than a new set of friction pegs and obviate the need for fine tuners.